Refactor legacy client to be independent of tokio

This patch decouples the functionality of `client::legacy` from tokio,
by lifting up a `Client`, `HttpConnection`, and `Resolver` into the
`client` module. For backwards compatibility, it reimplements
`client::legacy` to be based off of the new code.

As best as I can tell it should be backwards compatible, and passes all
the tests.

There's a couple of areas where I'm not sure if we got quite the right API:

* Capturing connection info - This adds a helper closure you can call to
  get it, which is then used by he legacy client to send the info to the
  tokio watch.
* This extracts the pool info, but there's been a long standing plan to
  replace it with something better. I'm not sure what that would look
  like though.
* I set up the legacy Client and HttpConnector to wrap the new generic
  APIs. That adds a bit of code duplication, but as far as I can tell we
  can't use a type alias due to the `new` method.

Author: erickt

Cargo.toml

@@ -71,12 +71,13 @@ full = [
     "http1",
     "http2",
     "tokio",
+    "tokio-net",
     "tracing",
 ]
 
-client = ["hyper/client", "tokio/net", "dep:tracing", "dep:futures-channel", "dep:tower-service"]
-client-legacy = ["client", "dep:socket2", "tokio/sync", "dep:libc", "dep:futures-util"]
-client-pool = ["client", "dep:futures-util", "dep:tower-layer"]
+client = ["hyper/client", "dep:futures-channel", "dep:futures-util", "dep:socket2", "dep:tracing", "dep:tower-service"]
+client-legacy = ["client", "tokio/sync", "dep:libc", "dep:futures-util"]
+client-pool = ["client", "dep:futures-util", "dep:tower-layer", "tokio/sync"]
 client-proxy = ["client", "dep:base64", "dep:ipnet", "dep:percent-encoding"]
 client-proxy-system = ["dep:system-configuration", "dep:windows-registry"]
 
@@ -89,7 +90,9 @@ service = ["dep:tower-service"]
 http1 = ["hyper/http1"]
 http2 = ["hyper/http2"]
 
-tokio = ["dep:tokio", "tokio/rt", "tokio/time"]
+tokio = ["dep:tokio", "tokio/time"]
+tokio-rt = ["dep:tokio", "tokio/rt"]
+tokio-net = ["tokio-rt", "tokio/net"]
 
 tracing = ["dep:tracing"]