draft-ietf-oauth-transaction-tokens-04: Merge pull request #150 from gffletch/rename-azd-119

Updates to the draft include:

• cleaning up text around the transaction context claim
• updates to transaction token service responsibility
• IANA text cleanup
• clarifications for the subject_token_type
• updated guidance regarding mutual authentication
• updates to the Txn-Token response
• updated security considerations and privacy considerations

Includes the following merged PRs:
#116, #122, #123, #125, #126, #136, #138, #139, #140, #141, #142, #143, #144, #146, #147, #148, #150