[mock-chronik-client] Add MockAgora to stub ts declaration

Summary:
mock-chronik-client has not been converted to ts yet. However, it has a stub ts declaration so that ts apps may use it without throwing an error.

Add MockAgora here so that ts apps using this do not get a linter error. mcc typescript conversion will happen eventually but meanwhile we should unbreak this.

Test Plan: npm test, proofread. I confirmed this stops the error in js apps running tslint.

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: #bitcoin_abc, johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D17274