Add ability to create a new WebPushClient from a custom Isahc/hyper c… #51

Job	Run time
clippy	35s
format	14s
cargo-test (stable, ubuntu-latest)	50s
cargo-test (stable, macos-latest)	45s
cargo-test (stable, windows-latest)	10m 7s
cargo-test (beta, ubuntu-latest)	49s
cargo-test (beta, macos-latest)	48s
cargo-test (beta, windows-latest)	7m 54s
cargo-test (nightly, ubuntu-latest)	57s
cargo-test (nightly, macos-latest)	1m 3s
cargo-test (nightly, windows-latest)	12m 43s
	36m 45s

Provide feedback