kTLS support

[bonifaido]

Description

Currently only the TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 cipher is implemented, this is the default cipher of BearSSL.

Fixes #54

Contains a lot of (untracked) other fixes and a small test that verifies that sendfile works with kTLS (via NGiNX).

TODO:

• add test for app without kTLS (I have added a test, but it works locally only, somehow not on GH Actions)
• add test with nginx (sendfile should be stressed)

Type of Change

• Bug Fix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass