[All SDKs] OAuth token endpoint should be configurable

[le-yams]

Description

For clients using OAuth2 credentials, the token endpoint is currently hardcoded in all SDKs (with /oauth/token value).
Could it be possible to make it configurable? Or even better support oidc discovery?

I'm willing to contribute if that's something you would be interested in :)

Steps to take

Change the apiTokenIssuer field in the configuration to accept a full URL.
So:

ApiTokenIssuer	Endpoint SDK will hit
issuer.fga.example	https://issuer.fga.example/oauth/token
https://issuer.fga.example	https://issuer.fga.example/oauth/token
https://issuer.fga.example:8080	https://issuer.fga.example:8080/oauth/token
issuer.fga.example/some_endpoint	https://issuer.fga.example/some_endpoint
https://issuer.fga.example/some_endpoint	https://issuer.fga.example/some_endpoint
https://issuer.fga.example:8080/some_endpoint	https://issuer.fga.example:8080/some_endpoint

Of course, we'll need to do some of the validations to ensure e.g. users are passing fields with https or http (and not e.g. ftp) and that the full url is valid

Related Issues

• .NET SDK issue: PathTemplate auth/token hardcoded in OAuth2Client ExchangeTokenAsync method dotnet-sdk#30
• (duplicate) SDKs should get the token exchange endpoint from the issuer's configuration #197

SDKs to be updated

• JS SDK (feat: support custom path in apiTokenIssuer js-sdk#139) by @marcoquotech
• Go SDK (feat(go-sdk): configurable client credentials token url #275)
• .NET SDK (feat: Allow the api token issuer to be a fully qualified url with a path component dotnet-sdk#151) by @OsmanMElsayed
• Python SDK Use token-endpoint instead of issuer for oauth2 authorization python-sdk#136
• Java SDK (feat(java-sdk): configurable token endpoint #240)

[le-yams]

I opened the PR #240 for the Java SDK. I have prepared all other SDKs (go, js, dotnet and python) but I'll wait your review on this one before submitting them 😃.