feat: add configurable JWT issuer for local auth development

[cemalkilic]

Summary

• Adds configurable jwt_issuer field to auth configuration in config.toml
• Updates startup to use configured JWT issuer when set
• Maintains backward compatibility by falling back to default API URL when not configured

Motivation

When developing OAuth integrations, the auth server OAuth discovery endpoint /.well-known/oauth-authorization-server returns URLs referencing localhost, which are inaccessible to third-party services trying to integrate with the local instance.

This change allows developers to configure the issuer url for the auth server, making it possible to test third-party OAuth integrations locally without deploying to a remote environment.

Relevant: https://github.com/orgs/supabase/discussions/38022#discussioncomment-14815820

[aantti]

This will be also configurable via GOTRUE_JWT_ISSUER env-var for self-hosted Supabase?

[cemalkilic]

Yes, similar to any other auth env variables, this will be also parsed by auth server 👍

[coveralls]

Pull Request Test Coverage Report for Build 19095672128

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 54.673%

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Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 19065008112:	-0.03%
Covered Lines:	6388
Relevant Lines:	11684

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💛 - Coveralls