Feature request: expose sendAs in client.sendMessage

[zeynalnia]

Problem

The TL schema for messages.sendMessage already supports send_as:flags.13?InputPeer
(api.tl, layer 193: messages.sendMessage#983f9745 ... send_as:flags.13?InputPeer ...),
but the high-level wrapper client.sendMessage does not expose this option. To send
a message as a channel, anonymous admin, or other peer, callers currently have to drop
down to client.invoke(new Api.messages.SendMessage(...)) manually.

Proposal

Add an optional sendAs?: EntityLike field to SendMessageParams in
gramjs/client/messages.ts. When provided, resolve it via client.getInputEntity
and forward to Api.messages.SendMessage({ sendAs: ... }).

Notes

• This mirrors how Pyrogram/Telethon expose the same option.
• Scope is intentionally limited to the no-file path; sendFile would need a separate
  change to thread sendAs through messages.SendMedia.