More powerful and configurable plugin system

[vladcalin]

The plugin system should rely on a BasePlugin class that will be inherited by all plugins.

Example

class ExamplePlugin(BasePlugin):
    name = "example"
    requires = [
        ConfigValue("db_engine", default="test"),
        ConfigValue("db_user", required=True),
        ConfigValue("db_password", required=True)
    ]
    
    def initialize(self):
        self.microservice.database = crate_db(
            self.get_config_value("db_engine"),
            self.get_config_value("db_user"),
            self.get_config_value("db_password")
        )
    def finalize(self, microservice):
        self.microservice.database.close()

The following method should be available for every plugin

• intialize - on service initialization
• finalize - on service shutdown
• on_method_call - called with a single method call
• on_method_response - called with a single response
• on_request - called for a user request (be it a single method call or a batch request)
• on_response - called for a http request
• on_internal_error - called for internal errors from methods
• on_jsonrpc_error - called for specific jsonrpc errors such as method not found, parse error, etc