Expose /v1/txn size limit in Go API client

Since there's a fixed 64-item limit, code often needs to know about this in order to chunk work, etc. We should expose a new endpoint that can get this size and make an efficient API call in the Go client to expose it (fetch it once and save it). This way code can be written in a generic way w/o having to hard-code the limit in the API client, since we may adjust the limit in the future.

The issue names the /v1/txn endpoint and the Go API client but no files or tests. Locate the endpoint handler and client entry points first; done means the limit is exposed through an endpoint and the client fetches it once and retains the value for chunking work.

Written by the indexing model from the issue text.