cherry-pick: use OpenResty's API for mtls (#99)

[bungle]

Summary

Cherry-picks:

• Added support for https_sni in healthcheck.lua (https://github.com/Kong/lua-resty-healthcheck/pull/49[)](https://github.com/Kong/lua-resty-healthcheck/commit/c8c6bb67803a3f36de13b486daea94072efcdd38)
• fix(mtls) use OpenResty's API for mtls (https://github.com/Kong/lua-resty-healthcheck/pull/99[)](https://github.com/Kong/lua-resty-healthcheck/commit/44b68d5893facde6c5232147cc1fc32f8430ec15)

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