Add flagr key

[zhouzhuojie]

Description

Support flagKey in the following endpoints

• GET /flags?key=<key>
• POST /flags (create flagKey, if empty, use a random one)
• PUT flags/<id> (update/save flagKey)
• POST /evaluation
• POST /evaluation/batch

Motivation and Context

• To support dedicated unique (semi-user-generated) key for the flags
• To support flag evaluation with obfuscated keys (e.g. exposing flagr to public)

How Has This Been Tested?

• UI changes to support flagKey
• Flagr evaluation api
• Flagr batch evaluation api
• Flagr migration
• No breaking changes

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.

Screenshots

[renjie-checkr]

Curious what's the reason to downgrade mysql?

[zhouzhuojie]

To match the mysql version of RDS, I was testing the migration locally with docker

[codecov-io]

Codecov Report

Merging #159 into master will decrease coverage by 0.47%.
The diff coverage is 86.3%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   93.37%   92.89%   -0.48%     
==========================================
  Files          25       25              
  Lines        1252     1309      +57     
==========================================
+ Hits         1169     1216      +47     
- Misses         56       63       +7     
- Partials       27       30       +3

Impacted Files	Coverage Δ
pkg/entity/fixture.go	100% <100%> (ø)	⬆️
pkg/handler/fixture.go	100% <100%> (ø)	⬆️
pkg/util/util.go	100% <100%> (ø)	⬆️
pkg/entity/flag.go	100% <100%> (ø)	⬆️
pkg/handler/eval_cache.go	88% <100%> (-1.29%)	⬇️
pkg/mapper/entity_restapi/e2r/e2r.go	100% <100%> (ø)	⬆️
pkg/entity/flag_snapshot.go	36.11% <25%> (-21.39%)	⬇️
pkg/handler/crud.go	93.87% <83.33%> (-0.43%)	⬇️
pkg/handler/eval.go	92.36% <91.3%> (+0.95%)	⬆️
pkg/mapper/entity_restapi/r2e/r2e.go	92% <0%> (-8%)	⬇️

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