Support a non-streaming interface for the agent monitor API

Currently, the only way to use the agent monitor API is via streaming HTTP. This is the ideal way to model log streaming, but older browsers (namely IE11) don't support streaming HTTP.

Our other streaming APIs have [follow, and offset params](https://www.nomadproject.io/api-docs/client#follow-1) that can used to construct a simple polling solution for log streaming while guaranteeing no messages lost between requests.

Since agent monitor right now is basically just teeing the agent logs, this probably isn't practical to build right now, but I think it should be a consideration when looking into #8053.

Start by reviewing the agent monitor API and the related work in issue #8053. Done would mean a non-streaming option that supports polling with follow and offset parameters, preserves messages between requests, and works for clients such as IE11.

Written by the indexing model from the issue text.