feat: supports http2

[lamweili]

superagent@8.x already has http2 support.
This PR enables the http2 support on supertest as well.

As this is a new feature (backward compatible and non-breaking), following semver, a minor version jump is required.

• Supersedes Support http2. #494
• Supersedes [feature] enable http2 support #691

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Tests are using Node.js native http2 module to create the server with 100% test coverage.

• The http2 tests are not using express (as express@4.x does not support http2)
• supertest should not be reliant on express anyway

[lamweili]

The tests are failing in the CI because of coveralls. Refer to PR #794.

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I commented the following away in my forked repository.

https://github.com/visionmedia/supertest/blob/e064b5ae71e1dfa3e1a74745fda527ac542e1878/.github/workflows/node.js.yml#L38-L40

The tests passed: https://github.com/lamweili/supertest/actions/runs/3177809614

[lamweili]

Thanks, @titanism!

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PS: You might want to close the following issues/PRs:

• Issue With http2 "socket hang up" #617
• PR Support http2. #494
• PR [feature] enable http2 support #691