Extend EvalCache export with a querying option

[iamafanasyev]

Description

Add a querying option to EvalCache export function in a #616 way:

• ids — csv of flag ids (highest precedence)
• keys — csv of flag keys (second precedence)
• enabled — queries flags with a given enabled status (could be combined with tags and all)
• tags — csv of flag tags to query (could be combined with enabled and all)
• all — whether to use ALL or ANY semantics over provided tags (ANY by default, could be combined with enabled and tags)

For example:

• GET /api/v1/export/eval_cache/json?tags=foo,bar&all=true to get only flags containing both foo and bar tags
• GET /api/v1/export/eval_cache/json?tags=foo&enabled=true to get only enabled flags containing foo tag
• GET /api/v1/export/eval_cache/json?ids=1,2 to get only flags of id 1 and 2
• GET /api/v1/export/eval_cache/json?keys=one,two to get only flags of keys one and two
• GET /api/v1/export/eval_cache/json?keys=one&tags=foo is equivalent to GET /api/v1/export/eval_cache/json?keys=one (tags=foo effectively would be omitted)

Motivation and Context

#628

How Has This Been Tested?

That is a new export functionality, so new tests/specs were added (TestEvalCacheExport)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 85.71429% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.16%. Comparing base (770461b) to head (0ee31f6).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/entity/fixture.go	0.00%	10 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #646      +/-   ##
==========================================
- Coverage   81.19%   77.16%   -4.04%     
==========================================
  Files          28       30       +2     
  Lines        2271     2067     -204     
==========================================
- Hits         1844     1595     -249     
- Misses        337      382      +45     
  Partials       90       90              

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