Refactor receiver webhook client to support custom dialers

[JacobsonMT]

• Introduced client options in HTTP client creation similar to upstream.
• Added support for custom DialContext in webhook client.
• Enhanced integration tests to validate custom dialer behavior.

Part of a series of PRs for full support. This PR lacks support for: mqtt, sns, slack, and prometheus-alertmanager. It also leaves more in-depth client refactoring to followups.

Most changes to tests are to facilitate the new factory_test that verifies a provided custom dialer blocking requests works as expected. Various issues with the "valid" testing configs for integrations were causing them to fail for other reasons before they hit the dialer.

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[yuri-tceretian]

I am not sure I understand where the hard-coded timeouts came from. Also, I am worried about coordination of hard-coded and configured timeouts. Other than that, LGTM