feat: enhance jsonschema for third_party config

[lfleischmann]

Description

The JSON schema for the third_party configuration currently does not accurately represent dependencies between properties. These changes address the following issues:

1. the schema validates against a configuration where any of the built-in providers or at least one provider in custom_providers is enabled but any one of third_party.redirect_url/third_party.error_redirect_url/third_party.allowed_redirect_urls is omitted - this should not happen, i.e. if a provider is enabled then third_party.redirect_url,third_party.error_redirect_url,third_party.allowed_redirect_urls are required
2. the schema does not validate against a configuration where any disabled custom_provider omits a property that would only be required if the provider was enabled (display_name, client_id, secret, scopes) - this should not happen, if a custom_provider is not enabled then all other configurations are optional

How to test

Generate the JSON schema for the configuration using the schema generate config cmd. Then do one of the following:

• let your IDE (tested with JetBrains GoLand) validate the schema against a configuration document (e.g. backend/config/config.yaml)
• use an online validator like https://www.jsonschemavalidator.net/ (you have to convert the config YAML to JSON for this one)