communicator/ssh: Add support SSH over HTTP Proxy (#30274)

Terraform's remote-exec provision hangs out when it execs on HTTP Proxy bacause it dosen't support SSH over HTTP Proxy. This commits enables Terraform's remote-exec to support SSH over HTTP Proxy.

* adds `proxy_*` fields to `connection` which add configuration for a proxy host
* if `proxy_host` set, connect to that proxy host via CONNECT method, then make the SSH connection to `host` or `bastion_host`

internal/communicator/shared/shared.go

@@ -72,6 +72,26 @@ var ConnectionBlockSupersetSchema = &configschema.Block{
 			Type:     cty.String,
 			Optional: true,
 		},
+		"proxy_scheme": {
+			Type:     cty.String,
+			Optional: true,
+		},
+		"proxy_host": {
+			Type:     cty.String,
+			Optional: true,
+		},
+		"proxy_port": {
+			Type:     cty.Number,
+			Optional: true,
+		},
+		"proxy_user_name": {
+			Type:     cty.String,
+			Optional: true,
+		},
+		"proxy_user_password": {
+			Type:     cty.String,
+			Optional: true,
+		},
 		"bastion_host": {
 			Type:     cty.String,
 			Optional: true,