feat: Request Tagging and more clear errors distinguish twin-origin

[sameh-farouk]

Note: should be merged after https://github.com/threefoldtech/tfgrid-sdk-go/tree/handle-back-pressure

Description

Added Request tags support in a backward-compatible way.

• Introduced a Tag type for well-known request tags.

• Added predefined tag TagFailFast to instruct relays to fail immediately if the destination is offline.

• New helper serializeTags() to normalize and encode tags into a comma-separated string.

• Added a new SendRequestWithTags method in Peer that allows sending requests with custom tags.

• Added new CallWithTags / CallWithSessionWithTags methods in RpcClient to allow specifying tags when making RPC calls.

Changes

• makeEnvelope now accepts a tags parameter and sets Envelope.Tags if provided.

• SendRequest refactored to delegate to SendRequestWithTags.

• SendResponse updated to use the new makeEnvelope signature (with tags="").

• RpcClient.CallWithSession now delegates to CallWithSessionWithTags.

Related Issues

• Add support to the request tagging #1425

Checklist

• Tests included
• Build pass
• Documentation
• Code format and docstring