server: use a whitelist for non-admin APIs

[knz]

This issue is to address the cleanup and strengthening of the solutions implemented for #42567

• the server code that accepts incoming RPCs and HTTP requests should consider that the request requires admin privileges by default, and only allow non-admins to use the endpoint if the endpoint is in a whitelist. This will prevent mistakenly introducing new privileged APIs without the appropriate privilege checks in the future. Action plan:

  • share the code that requires admin privileges to the top level incoming request code
  • introduce a whitelist

• the auth info should be determined very early (during the TLS handshake) and populated in the context for use everywhere, see these two comments:

#42567 (comment)

and

#42567 (comment)

Epic: CRDB-1473

Jira issue: CRDB-5247

[knz]

@dhartunian this is one of the issues we discussed earlier today, if you want to keep an eye on it or add it to some to-do list.

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