Can we ditch support for <SecretKey> in credentials?

[Rod Widdowson]

The security schema supports one or more <SecretKey> elements inside the
xsi:type="BasicCredential". 

In V2 we parse the first one of these (silently dropping the rest on the
floor), but as soon as we try to make some sense of these (prior to
inserting them into a BasicCredential) the support code throws an exception
because that is not yet supported.

In V3 the support code is in the same state.

I'm not an expert in this domain and so I just don't have the insight into
the requirements.  So the question is whether I should duplicate the V3
parsing (allowing us to bolt in SecretKey support at a later date without
having to jiggle the parsers), or whether it suffices to issue a log.WARN if
we discover such a declaration? Or both?

/R