Make HighlightBuilder implement Writable

[cbuescher]

In the current state of the search refactoring the SearchSourceBuilder still holds the highlighter as BytesReference which needs to be parsed later on the shard. This was done to speed up the query refactoring but in order to parse it on the coordinating, we need to make HighlightBuilder and its nested structures (mainly Field) to implement Writeable and override equals and hashcode for testing.

Relates to #10217

[cbuescher]

Current plan is to split this in three different PRs:

• make HighlightBuilder implement Writable, add equals(), hashCode() and tests (Make HighlighterBuilder implement Writable, equals and hashCode #15113)
• add fromXContent method to HighlightBuilder, using same parsing logic as in the current SearchContextHighlighter ( Add fromXContent method to HighlightBuilder #15157)
• create conversion method from HighlightBuilder to SearchContextHighlight, since that is what we still use in the SearchContext (Enable HighlightBuilder to create SearchContextHighlight #15324)

EDIT:
There are some additional issues that should be adressed before this issue is closed:

• Make parsing of global highlight parameters and field-level overwrites more consistent (Joint parsing of common global Hightlighter and subfield parameters #15368)
• use HighlightBuilder in SearchSourceBuilder (Use HighlightBuilder in SearchSourceBuilder #15376)