[feat](protocol, client, server): replace DTLS with gRPC for tunnel implementation

- Introduced gRPC-based tunnel design for bi-directional communication, replacing legacy DTLS transport.
- Added `HopGateTunnel` gRPC service with client and server logic for `OpenTunnel` stream handling.
- Updated client to use gRPC tunnel exclusively, including experimental entry point for stream-based HTTP proxying.
- Removed DTLS-specific client, server, and related dependencies (`pion/dtls`).
- Adjusted `cmd/server` to route gRPC and HTTP/HTTPS traffic dynamically on shared ports.

go.mod

@@ -7,9 +7,9 @@ require (
 	github.com/go-acme/lego/v4 v4.28.1
 	github.com/google/uuid v1.6.0
 	github.com/lib/pq v1.10.9
-	github.com/pion/dtls/v3 v3.0.7
 	github.com/prometheus/client_golang v1.19.0
 	golang.org/x/net v0.47.0
+	google.golang.org/grpc v1.76.0
 	google.golang.org/protobuf v1.36.10
 )
 
@@ -20,15 +20,13 @@ require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/bmatcuk/doublestar v1.3.4 // indirect
 	github.com/cenkalti/backoff/v5 v5.0.3 // indirect
-	github.com/cespare/xxhash/v2 v2.2.0 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/go-jose/go-jose/v4 v4.1.3 // indirect
 	github.com/go-openapi/inflect v0.19.0 // indirect
 	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/hashicorp/hcl/v2 v2.18.1 // indirect
 	github.com/miekg/dns v1.1.68 // indirect
 	github.com/mitchellh/go-wordwrap v1.0.1 // indirect
-	github.com/pion/logging v0.2.4 // indirect
-	github.com/pion/transport/v3 v3.0.7 // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/prometheus/common v0.48.0 // indirect
 	github.com/prometheus/procfs v0.12.0 // indirect
@@ -41,4 +39,5 @@ require (
 	golang.org/x/sys v0.38.0 // indirect
 	golang.org/x/text v0.31.0 // indirect
 	golang.org/x/tools v0.38.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251022142026-3a174f9686a8 // indirect
 )