docs(hosts,hop): add doc comments, fix missing httpLoader.Close, accumulate reload errors

- hosts/hosts.go: add package and symbol doc comments, fix missing
  httpLoader.Close() in Close()
- hosts/plugin: add doc comments for HTTP and gRPC plugin types
- hop/hop.go: add doc comments, accumulate reload/parse errors via errors.Join
  instead of silently discarding them
- hop/plugin/http.go: add doc comments and missing Close() method
- Add unit tests for hosts (580 lines) and hop (991 lines)

hop/plugin/http.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"io"
 	"net/http"
 
 	"github.com/go-gost/core/chain"
@@ -115,3 +116,14 @@ func (p *httpPlugin) Select(ctx context.Context, opts ...hop.SelectOption) *chai
 	}
 	return node
 }
+
+func (p *httpPlugin) Close() error {
+	if p.client == nil {
+		return nil
+	}
+	p.client.CloseIdleConnections()
+	if tr, ok := p.client.Transport.(io.Closer); ok {
+		return tr.Close()
+	}
+	return nil
+}