Add support for per-target options

[jhump]

This also refactors how "keep warm" targets are determined: basically, any explicitly configured backend target is kept warm. This also adds a new option, WithDisallowUnconfiguredTargets, which means the client will refuse to service requests to unknown backend targets.

When moving things around to handle the fact that many options are now per-target, I realized the applyRequestTimeout I had added was never called. When I actually revised the code to use it, I've encountered a conundrum: I've got a cancel function, but don't know when to call it. To prevent resource leaks, it should be called when the operation completes (to release the timer goroutine that is otherwise scheduled to cancel the context if it takes too long). So... I added a rudimentary/preliminary form of "detecting when the request is done", and went ahead and also use that to track the number of active requests per leaf transport (which we'll need for least-loaded load balancing anyway). That makes this the start of TCN-1867.

Resolves TCN-1878

[linear]

TCN-1878 Add ability to configure some options on a per-backend basis

In addition to defining "defaults" (when configuration is uniform), we'll need a way to configure many of the options on a per-backend basis. A good option is to provide a second kind of option like BackendOption (in addition to ClientOption), and the options that can vary per-backend can implement that interface, too. That way they can be provided as ClientOption and used as a global/default, but also used in something like the following, allowing users to per-backend overrides:

func WithBackendOptions(scheme, hostPort string, opts ...BackendOption) ClientOption {
    // implement me
}

Using WithBackendOptions could also automatically add the given scheme and host:port to the set of "keep warm" targets, too.

[jchadwick-buf]

A few comments, most importantly a concern about how to overwrite Response.Body.

[jchadwick-buf]

Yeah, that seems fine too. A little verbose, but not so bad.