Add a ResolveNow method for resolvers

[jchadwick-buf]

• Rename Resolver to Factory, and create Resolver interface for resolver processes.
• Add ResolveNow() method.
• Add test that tests that ResolveNow() works. Note that this new test moves the resolver tests to using channels a la the healthchecker test. This is needed to avoid racy behavior, otherwise our refresh signal races with the timer being unblocked. If the refresh signal is not quick enough, the timer will remain blocked when BlockUntilContext is called and cause the test assertions to run too early. A channel elegantly solves this.

[linear[bot]]

TCN-1982 Update resolver interfaces to support "ResolveNow" operation

Currently, starting a resolve process just returns an `io.Closer`. It should instead return a value that also has a `ResolveNow()` method. We should probably rename `resolver.Resolver` to `resolver.Factory` and the new interface (with `Close() error` and `ResolveNow()` methods) can be called `resolver.Resolver`.