feat(admin): add support for IssuerURL in OAuth2 flow

[freeznet]

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[github-actions]

@freeznet:Thanks for your contribution. For this PR, do we need to update docs?
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[Copilot]

Pull request overview

This PR adds support for the IssuerURL field in the OAuth2 ClientCredentialsFlow configuration for Pulsar Admin clients. The change ensures that the issuer endpoint is properly passed to both the Issuer struct and the ClientCredentialsFlowOptions struct when creating OAuth2 authentication providers.

Changes:

• Added IssuerURL field to ClientCredentialsFlowOptions initialization, setting it to the same value as IssuerEndpoint
• Introduced a package-level variable newAuthenticationOAuth2WithFlow for better testability
• Added comprehensive unit tests to verify the OAuth2 configuration is correctly passed through

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pkg/admin/interface.go	Added package variable for testability and set IssuerURL field in OAuth2 flow options
pkg/admin/interface_test.go	Added new test file with unit tests verifying OAuth2 IssuerURL configuration

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