fix: (DeclarativeOAuthFlow) - add access_token_params property to match the actual spec properties

[bazarnov]

What

While working on the migration for TikTok Marketing OAuthFlow, there was a need to encode the access_token_url query parameters to json string instead of url_encoded parameters.

The access_token_params is there to provide such a functionality, in order to match the actual spec provided with the designed protocol DeclarativeOAuth spec this property should exist at the protocol level.

How

• add the access_token_params object property to the OAuthConfigSpecification.oauth_connector_input_specification.properties
• made the additionalProperties set to true to allow future lazy spec improvements, even though the protocol could be behind of the actual source / destination spec, allowing us to provide more properties to work with.