Add Support for Nested Metadata Configuration in OAuth2 Client Using JSON Format

[YevheniiSemenko]

This update introduces the ability to configure the metadata field in OAuth2 clients using the metadata_json parameter, allowing for nested key-value structures.

The current implementation only supported flat key-value pairs, which limited use cases requiring more complex metadata. With the new approach, users can specify nested metadata by providing it in JSON format.

The change is backward-compatible, ensuring that existing users relying on the metadata field without JSON can continue without disruption. Both terraform apply and terraform import handle the new format seamlessly.

During terraform import, the provider detects nested metadata and stores it in metadata_json if needed, while simple structures remain in metadata.

Example of usage:

resource "hydra_oauth2_client" "client_with_metadata_json" {
	client_name = "client_with_metadata_json"
	metadata_json = jsonencode({
		"nested" = {
			"key" = "value"
		},
		"first_party" = true
	})
	redirect_uris = ["http://localhost:8080/callback"]
	response_types = ["code"]
	token_endpoint_auth_method = "none"
}

[YevheniiSemenko]

I tweaked the GitHub Actions a bit since some things were recently deprecated, causing the test and release pipelines to fail.

Also, I hope you won’t be offended, but I registered my fork in the Terraform registry. We're using Atlantis to manage Terraform configurations, and having a custom provider means we would need to release a custom Atlantis image. For a secure environment, that’s a lot of hassle and requires much more effort. Once the PR is merged, I'll request the deletion of the fork. Thank you for understanding!

[svrakitin]

LGTM, thanks for the contribution.

I'm happy with people forking stuff, I don't have much time to dedicate to this provider as I'm not using Hydra currently. :)