Make `ClientRequestContext.authority()` and `host()` return non-null

[trustin]

ClientRequestContext.authority() and host() may return null until the context is fully initialized. I believe there's only a small fraction of situation where a partially initialized context is available to a user. Could we make them non-null and raise an exception instead of returning null?

There are two cases a user may see a partially initialized context:

• A user retrieves the ClientRequestContext via Clients.newContextCaptor() and withContextCustomizer().
• EndpointGroup.selectNow() and select()

[erie0210]

I'd like to work on this issue if it's okay.

[jrhee17]

Sure, go for it