Add a dedicated client to communicate with the Proxy SSH server (grav…

…itational#22629)

A new `api/client/proxy/Client` has been added to interact with
the SSH and gRPC servers that the Proxy serves on its SSH port.
The client will first try connecting to the gRPC server and if
that fails it will fall back to the SSH server.

Much of the SSH functionality mimics the existing behavior of the
`ProxyClient` in `lib/client`. This is the first part of phasing
out that client in favor of the new client. There will be a follow
up PR that migrates `lib/client` to make use of the new client instead.

Part of gravitational#19812

api/client/contextdialer.go

@@ -26,11 +26,11 @@ import (
 	oteltrace "go.opentelemetry.io/otel/trace"
 	"golang.org/x/crypto/ssh"
 
-	"github.com/gravitational/teleport/api/client/proxy"
 	"github.com/gravitational/teleport/api/client/webclient"
 	"github.com/gravitational/teleport/api/constants"
 	"github.com/gravitational/teleport/api/observability/tracing"
 	tracessh "github.com/gravitational/teleport/api/observability/tracing/ssh"
+	"github.com/gravitational/teleport/api/utils"
 	"github.com/gravitational/teleport/api/utils/sshutils"
 )
 
@@ -81,7 +81,7 @@ func tracedDialer(ctx context.Context, fn ContextDialerFunc) ContextDialerFunc {
 func NewDialer(ctx context.Context, keepAlivePeriod, dialTimeout time.Duration, opts ...DialProxyOption) ContextDialer {
 	return tracedDialer(ctx, func(ctx context.Context, network, addr string) (net.Conn, error) {
 		dialer := newDirectDialer(keepAlivePeriod, dialTimeout)
-		if proxyURL := proxy.GetProxyURL(addr); proxyURL != nil {
+		if proxyURL := utils.GetProxyURL(addr); proxyURL != nil {
 			return DialProxyWithDialer(ctx, proxyURL, addr, dialer, opts...)
 		}
 		return dialer.DialContext(ctx, network, addr)
@@ -116,7 +116,7 @@ func NewProxyDialer(ssh ssh.ClientConfig, keepAlivePeriod, dialTimeout time.Dura
 func newTunnelDialer(ssh ssh.ClientConfig, keepAlivePeriod, dialTimeout time.Duration, opts ...DialProxyOption) ContextDialer {
 	dialer := newDirectDialer(keepAlivePeriod, dialTimeout)
 	return ContextDialerFunc(func(ctx context.Context, network, addr string) (conn net.Conn, err error) {
-		if proxyURL := proxy.GetProxyURL(addr); proxyURL != nil {
+		if proxyURL := utils.GetProxyURL(addr); proxyURL != nil {
 			conn, err = DialProxyWithDialer(ctx, proxyURL, addr, dialer, opts...)
 		} else {
 			conn, err = dialer.DialContext(ctx, network, addr)