feat: Support for http/2

[nabetti1720]

Issue #338 (if available)

Description of changes

• If the environment variable LLRT_HTTP_VERSION is not present, the connection is established with http/2. (This is the default behavior.)
• If the environment variable LLRT_HTTP_VERSION is set to 1.1, the connection is established with http/1.1.
• If the environment variable LLRT_HTTP_VERSION is set to a value other than 1.1 (for example, 5), the connection is established with http/2.

Checklist

• [x] Created unit tests in tests/unit and/or in Rust for my feature if needed
• [x] Ran make fix to format JS and apply Clippy auto fixes
• [x] Made sure my code didn't add any additional warnings: make check
• [x] Updated documentation if needed (API.md/README.md/Other)

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