[keyserver] replace farcaster bot reply text with frame

Summary:
This replaces the farcaster bot reply text with a frame

Depends on D14092

Test Plan:
I tested by using ngrok with my local keyserver and triggering a webhook event on warpcast. This caused the farcaster bot to properly reply with the frame.
Made sure that the join channel button on the frame properly redirected to a comm app invite link

Before landing I plan on testing this on my self-hosted keyserver

(It's replying as "wyilio" because I used my farcaster account to send the cast`)

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14093