This is the SNI sent in the client TLS connection to a backend.

We use VMOD dynamic for backends represented by an ExternalName
Service (likely the common use case for TLS onload). VMOD dynamic
does not have the authority field that klarlack makes available
for standard backends. But if the host_header field is set for
a VMOD dynamic director, the VMOD uses that value for the SNI.

So if the BackendConfig authority field is set, we also assign its
value to the host_header field. Since BackendConfig also has a
separate field for host_header, both of them could be conceivably
set to different values. If we find that the two fields are set
to non-empty, conflicting values, the controller emits a
SyncFatalError, and the BackendConfig is not synced.