[Tunnelbroker] rename WebPushConfig to camelCase

Summary: We have a prod config in camelCase style.

Test Plan: Tested with @bartek's stack - the same secrets we have on AWS KMS are correctly parsed.

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, bartek

Differential Revision: https://phab.comm.dev/D12891