Add Go SDK implementation

[Hyodar]

Hi there!

We're now using Dstack in some of our TEE experiments in Nethermind - more recently https://github.com/NethermindEth/teeception!
The codebase is in Go, so we implemented a quick Go Dstack SDK based on the existing TS and Python implementations. If that's interesting to you, happy to contribute here. Feel free to let me know if there's anything else needed.

This PR adds a Go SDK implementation effectively equivalent to the TS and Python ones, as well as a simple README and unit tests.

Using it is mostly similar to the other ones as well - here's an example:

package main

import (
	"context"
	"fmt"
	"log/slog"

	"github.com/Dstack-TEE/dstack/sdk/go/tappd"
)

func main() {
	client := tappd.NewTappdClient(
		// tappd.WithEndpoint("http://localhost"),
		// tappd.WithLogger(slog.Default()),
	)

	deriveKeyResp, err := client.DeriveKey(context.Background(), "/", "test", nil)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(deriveKeyResp) // &{-----BEGIN PRIVATE KEY--- ...

	tdxQuoteResp, err := client.TdxQuote(context.Background(), []byte("test"), tappd.SHA256)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(tdxQuoteResp) // &{0x0000000000000000000 ...

	rtmrs, err := tdxQuoteResp.ReplayRTMRs()
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(rtmrs) // map[0:00000000000000000 ...
}

[kvinwang]

Thank you for the awesome implementation. Looks excellent overall, with one minor note. @Leechael, please review.

[Leechael]

btw please remember to include // Author: at the top of the source code file.