Switch the "CLAIM_DEF" label to be "CRED_DEF"

[swcurran]

We've been trying to remove the term "CLAIM_DEF" in favour of "CRED_DEF" for quite awhile. While I realize it is still in some of the Indy code, it would be nice to banish the term in public places. IndyScan is the most of public of places for Indy Networks, so it would be really nice to have the label changed to the more appropriate "CRED_DEF".

In the work we are doing in creating the specification for AnonCreds (here: https://anoncreds-wg.github.io/anoncreds-spec/) we are only using the term "CRED_DEF", with a note that in some places in the code it is "CLAIM_DEF".

Thanks!