Add support for `omitempty` field tag to exclude empty values from queries and headers

[alecsammon]

Adds support for an omitempty tag.

Main use case is for query parameters where we may not want have empty keys

i.e. /pets?name_pointer=

By adding omitempty to the tag then this the key will not appear in the query string

i.e.

	type SearchQuery struct {
		NamePointer *string `in:"query=name_pointer,omitempty"`
	}

This will create /pets

This has been added to the formencoder so that it will also remove empty headers.

I have picked omitempty as I feel this will feel most natural for engineers as they are likely already used to this syntax with JSON.

[alecsammon]

Build is failing due to rate limiting in CodeCov - will try a rebuild later.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 90.70%. Comparing base (6a628af) to head (a7e1cb0).

Files	Patch %	Lines
core/omitempty.go	50.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   90.78%   90.70%   -0.08%     
==========================================
  Files          34       35       +1     
  Lines        1530     1539       +9     
==========================================
+ Hits         1389     1396       +7     
- Misses         97       99       +2     
  Partials       44       44              

Flag	Coverage Δ
unittests	90.70% <77.77%> (-0.08%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.