GH-4492 new transaction endpoints

[abrokenjester]

GitHub issue resolved: #4492

Briefly describe the changes proposed in this PR:

• separate endpoints for each txn operation
• backward compatible client impl
• updated openapi spec, added changelog

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PR Author Checklist (see the contributor guidelines for more details):

• my pull request is self-contained
• I've added tests for the changes I made
• I've applied code formatting (you can use mvn process-resources to format from the command line)
• I've squashed my commits where necessary
• every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change

[abrokenjester]

The more I stare at it, the more I'm wondering if this change is actually worth it. I'm going to park this for a bit, get back to it later.