related to influxdata/telegraf#14090

This PR introduces a new Option, HTTPClient, allowing users to override the *http.Client used by the Sender.

This allows a variety of customization options, such as proxy settings, idle connection management, and TLS Options. It is intended to integrate well with Telegraf's http config, which can generate an http.Client from a standard toml configuration (https://github.com/influxdata/telegraf/blob/master/plugins/common/http/config.go)

This PR reimplements the existing Timeout and TLSConfig options in terms of configuration.HTTPClient, and eliminates the internal NewClient factory.

feedback request: the PR logs a warning if you try to use more than one of Timeout, TLSConfig, and HTTPClient for the same Sender, because subsequent calls will clobber the first call. Curious if anyone thinks this should work differently.

I had a change of heart about my initial solution to mixing Timeout, TLSConfig, and HTTPClient. I just pushed a second commit with a different solution. This allows Timeout and TLSConfig to be used together, but prints a warning if either are used after HTTPClient. The intended logic now is that if you set an HTTPClient, the Sender will just use that, but you can alternatively provide either, both, or neither of Timeout and TLSConfigOptions as easier shorthand methods for changing those two settings. In that case, we'll make an http.Client for you.