Support headers for `http-proxy-agent`

[jportner]

Currently, HttpProxyAgent does not have a headers option at all. The HttpsProxyAgent does, though.

This PR adds a headers option, allowing consumers to set the proxy headers statically or dynamically.

Companion PR to update HttpsProxyAgent: #174

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 301862fc6e20c74945bdfabc9ce1c76143588966

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ---------------- | ----- | | http-proxy-agent | Minor |

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Name	Status	Preview	Updated (UTC)
proxy-agents	✅ Ready (Inspect)	Visit Preview	May 18, 2023 0:36am

[TooTallNate]

Thanks!