Allow clients to configure httpClient, e.g. set a pooled/keep-alive
client.
When caller configures HttpClient explicitly, we aim to use as-is; e.g.
we assume it's configured with TLS already. Expose `ConfigureTLS` to
aid api consumers with configuring their http client.
Also, removes `SetTimeout` call that I believe is internal only and has
odd side-effects when called on already created config. Also deprecates
`config.ConfigureTLS` in preference to the new `ConfigureTLS`.