add support for automatically parsing headers from the value during encode

[rnantes]

Description

• add support for automatically parsing headers from the value during encode
• add KeyEncodingStrategy and KeyDecodingStrategy
• add TimeZone and encoding decoding and Strategy
• add swift-collections dependency for OrderedSet implementation

Checklist

The following list must only be fulfilled by code-changing PRs. If you are making changes on the documentation, ignore these.

• Include in-code documentation at the top of the property/function/structure/class (if necessary).
• Merge to develop.
• Add to existing tests or create new tests (if necessary).

[rnantes]

@dehesa can you review this PR. it would be good to get upstream