Changelog for ngx-export-healthcheck-1.6

Added support for https transport protocol.
- Breaking changes: endpoint configuration gets a new field epProto, see updated examples in the project's README.md.
A new approach to building custom handlers with Cabal Nix-style local builds and ngx-export-distribution, see details in the project's simple/, periodic/, and prometheus/ subdirectories.
Improvement: using a single http manager for all service keys.