PUT against /txn results in http 413, make configurable please

I'm dong a PUT against /txn w/ about 80 KV entries and I get a vauge 413 back.

None of my keys are > 512 bytes. I'm doing this intentionally, and I'm not abusing the system, despite this:

https://github.com/hashicorp/consul/blob/8beb7164143397915f3487d95d9f810a9102a82c/agent/txn_endpoint.go#L16-L21

Please make this configurable, otherwise it sort of defeats the purpose of a "transaction" when now I have to manage yet another transaction outside of calling `/txn`

Start with agent/txn_endpoint.go at the linked lines and trace how the transaction request-size limit produces HTTP 413. Identify the existing configuration path for this limit, if one is present, and determine the endpoint behavior that should become configurable. Done means callers can set the limit without managing multiple transactions and oversized requests still receive the intended response.

Written by the indexing model from the issue text.