Add sendAs option to client.sendMessage

[zeynalnia]

Closes #827.

Summary

Exposes the send_as parameter (already present on messages.sendMessage since
layer 167) through the high-level client.sendMessage wrapper, so callers no
longer need to drop down to client.invoke(new Api.messages.SendMessage(...))
just to post on behalf of a linked channel or anonymous-admin identity.

Changes

• gramjs/client/messages.ts
  • Add sendAs?: EntityLike to SendMessageParams with JSDoc.
  • Resolve it via client.getInputEntity(sendAs) alongside the target entity.
  • Forward the resolved InputPeer to both Api.messages.SendMessage
    constructions (the Api.Message-resend branch and the fresh-text branch).
• __tests__/client/sendMessage.spec.ts (new)
  • Two specs against a stub TelegramClient: (1) asserts sendAs is resolved
    and forwarded onto the constructed request, (2) asserts the field is left
    undefined when the option is omitted.

Scope

Limited to the no-file path. The file branch delegates to client.sendFile,
which would need a separate change to thread sendAs through messages.SendMedia.

Test plan

• npx jest __tests__/client/sendMessage.spec.ts — both specs pass locally.
• Manual smoke test against a Telegram account with permission to post as a
  linked channel.