Support OAuth protected registry server

[JasonYeMSFT]

What does this PR do?

[Provide a clear, concise description of the changes]

This PR extends the RegistryServerProvider to support adding registry servers protected by OAuth for all its MCP accesses.

[Any additional context, screenshots, or information that helps reviewers]

registry.json now let's MCP servers with http transport to optionally declare an oauthScopes parameter. When this property is defined, the RegistryServerProvider will instantiate the corresponding McpClient object with a custom HttpClient. This custom HttpClient object has a delegate handler that fetches an access token and sets it in the Authorization header in each request.

Text scenarios

• local stdio + environment credential
• local stdio + user credential
• remote + environment credential
• remote + user credential obo

The server may apply additional restrictions that limit access, such as the tenant restriction.

GitHub issue number?

Pre-merge Checklist

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