Retry logic for schema registry appears broken

[steven-hubbard]

POSTs sent as part of a retry contain no body. Steps to reproduce:

1. Create a changefeed to a kafka sink using a schema registry
2. CRDB goes to register a schema
3. The schema registry returns 200 but only the ID due to an invalid body (due to a malformed request)
4. CRDB issues retries but all subsequent retries only contain the ID, no body.

I think retries here are broken because the first attempt consumes the bytes.Buffer, so subsequent attempts are made with an empty POST body

Jira issue: CRDB-13618

Epic CRDB-11783

[miretskiy]

I think retries here are broken because the first attempt consumes the bytes.Buffer, so subsequent attempts are made with an empty POST body

I don't think that's true. http Post creates new reader for the buffer.

[blathers-crl]

cc @cockroachdb/cdc

[jayshrivastava]

Fixed by #98135