feat: Add `InsecureRequestBuilder` for emulator

[pulzarraider]

fixes https://github.com/googleapis/google-cloud-php/issues/7187

According to documentation, the Pub/Sub emulator only supports unsecure (HTTP) connections. After updating google/cloud-pubsub to version 2 there was no option to use the emulator with REST transport (without gRPC PHP extension). While the fix in https://github.com/googleapis/google-cloud-php/pull/7588 provided some assistance, it still required a significant amount of code to modify the client to generate URLs with the http protocol.

This merge request introduces InsecureRequestBuilder which automatically generates URLs with the http scheme, simplifying the process.