Cleanup: de-duplicate auth_param realm configuration

Cleanup: de-duplicate auth_param realm configuration

Move realm parse and config dump logics to Auth::Config base object.
This de-duplicates Basic, Digest (and future schemes ie Bearer) config
processing code.  Also makes realm available to NTLM and Negotiate
schemes, although at present it remains unused by those schemes.

Also, convert the realm parameter string to an SBuf. Removing the need
for some memory maintenance code.