feature: HTTP client option for OAuthHandler

[logorrheic]

Problem Statement

transport.OAuthHandler doesn't seem to support setting a custom http.Client.

Proposed Solution

Inherit the http.Client from the StreamableHTTP (which does support this) and/or allow direct override in client.GetOAuthHandler.

Thanks for mcp-go!

[appleboy]

See #442

[logorrheic]

Thank you!