[keyserver] implemented FarcasterChannelTagBlob type for upload payload with validator

Summary:
Made a type for the FarcasterChannelTagBlob upload payload with a validator.

Later in the stack, I'll be downloading this blob and will need to validate to ensure proper type checking

Depends on D13773

Test Plan: flow check

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: bartek, tomek

Differential Revision: https://phab.comm.dev/D13774