expose trigger configuration schema

Note that this commit contains the following breaking changes:

* remote pull and remote push triggers are now enabled by default (with
  empty target list)
* remote pull and remote push triggers now require target option to be
  set (old behaviour had fallback on `gitremote`)
* validation is now considered to be stable, so it is enabled by default
  in docker image (can be disabled however)

src/ahriman/core/triggers/trigger.py

@@ -17,9 +17,12 @@
 # You should have received a copy of the GNU General Public License
 # along with this program. If not, see <http://www.gnu.org/licenses/>.
 #
-from typing import Iterable
+from __future__ import annotations
+
+from typing import Iterable, List, Optional, Type
 
 from ahriman.core.configuration import Configuration
+from ahriman.core.configuration.schema import ConfigurationSchema
 from ahriman.core.log import LazyLogging
 from ahriman.models.package import Package
 from ahriman.models.result import Result
@@ -30,6 +33,9 @@ class Trigger(LazyLogging):
     trigger base class
 
     Attributes:
+        CONFIGURATION_SCHEMA(ConfigurationSchema): (class attribute) configuration schema template
+        CONFIGURATION_SCHEMA_FALLBACK(Optional[str]): (class attribute) optional fallback option for defining
+            configuration schema type used
         architecture(str): repository architecture
         configuration(Configuration): configuration instance
 
@@ -47,10 +53,13 @@ class Trigger(LazyLogging):
             >>> configuration = Configuration()
             >>> configuration.set_option("build", "triggers", "my.awesome.package.CustomTrigger")
             >>>
-            >>> loader = TriggerLoader("x86_64", configuration)
+            >>> loader = TriggerLoader.load("x86_64", configuration)
             >>> loader.on_result(Result(), [])
     """
 
+    CONFIGURATION_SCHEMA: ConfigurationSchema = {}
+    CONFIGURATION_SCHEMA_FALLBACK: Optional[str] = None
+
     def __init__(self, architecture: str, configuration: Configuration) -> None:
         """
         default constructor
@@ -62,6 +71,60 @@ class Trigger(LazyLogging):
         self.architecture = architecture
         self.configuration = configuration
 
+    @classmethod
+    def configuration_schema(cls: Type[Trigger], architecture: str,
+                             configuration: Optional[Configuration]) -> ConfigurationSchema:
+        """
+        configuration schema based on supplied service configuration
+
+        Notes:
+            Schema must be in cerberus format, for details and examples you can check built-in triggers.
+
+        Args:
+            architecture(str): repository architecture
+            configuration(Optional[Configuration]): configuration instance. If set to None, the default schema
+                should be returned
+
+        Returns:
+            ConfigurationSchema: configuration schema in cerberus format
+        """
+        if configuration is None:
+            return cls.CONFIGURATION_SCHEMA
+
+        result: ConfigurationSchema = {}
+        for target in cls.configuration_sections(configuration):
+            if not configuration.has_section(target):
+                continue
+            section, schema_name = configuration.gettype(
+                target, architecture, fallback=cls.CONFIGURATION_SCHEMA_FALLBACK)
+            if schema_name not in cls.CONFIGURATION_SCHEMA:
+                continue
+            result[section] = cls.CONFIGURATION_SCHEMA[schema_name]
+
+        return result
+
+    @classmethod
+    def configuration_sections(cls: Type[Trigger], configuration: Configuration) -> List[str]:
+        """
+        extract configuration sections from configuration
+
+        Args:
+            configuration(Configuration): configuration instance
+
+        Returns:
+            List[str]: read configuration sections belong to this trigger
+
+        Examples:
+            This method can be used in order to extract specific configuration sections which are set by user, e.g.
+            from sources::
+
+                >>> @staticmethod
+                >>> def configuration_sections(cls: Type[Trigger], configuration: Configuration) -> List[str]:
+                >>>     return configuration.getlist("report", "target", fallback=[])
+        """
+        del configuration
+        return []
+
     def on_result(self, result: Result, packages: Iterable[Package]) -> None:
         """
         trigger action which will be called after build process with process result