Sync vMCP CRD auth implementation with the server (or vice versa)

[jhrozek]

The VMCP outgoing auth CRDs have not been designed in sync with the actual implementation

The issues are:

1. The discovered mode is not implemented in vMCP at all
2. ExtenalAuthConfig doesn't seem to be resolved - the controller seems to use it to populate the status.discoveredBackEnds but I can't find code that actually does anything with the external auth
3. (biggest) The auth strategies are not compatible. The vMCP implementation uses unauthenticated, header_injection and token_exchange. For token_exchange we have an equivalent in the MCPExternalAuthConfig CRD, but not for the first two. We should decide whether to keep adding to MCPExternalAuthConfig or fork out a special CR for vMCP.