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`.

`*Config.ConfigureTLS()` is invoked internally by `NewClient` and API
consumers should not invoke directly.

Now that http client is created in `api.NewClient`,
`*Config.ConfigureTLS` makes no sense.  API consumers that call it
explicitly can remove the invocation and preserve the behavior.