Add benchmark

[zhouzhuojie]

Description

Add go test -bench for critical flag evaluation code path, so that we can track of performance regression.

Motivation and Context

How Has This Been Tested?

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.

[zhouzhuojie]

Some result:
https://app.circleci.com/pipelines/github/checkr/flagr/68/workflows/602bf13e-fd01-4cd3-87fe-59b28b8cf1f5/jobs/1482/parallel-runs/0/steps/0-105

PASS
ok  	github.com/checkr/flagr/pkg/config	0.103s
PASS
ok  	github.com/checkr/flagr/pkg/entity	0.277s
goos: linux
goarch: amd64
pkg: github.com/checkr/flagr/pkg/handler
BenchmarkEvalFlag-36           	   40384	     29396 ns/op	    1640 B/op	      36 allocs/op
BenchmarkEvalFlagsByTags-36    	   38822	     31579 ns/op	    1647 B/op	      35 allocs/op
PASS
ok  	github.com/checkr/flagr/pkg/handler	3.308s
?   	github.com/checkr/flagr/pkg/mapper/entity_restapi/e2r	[no test files]
?   	github.com/checkr/flagr/pkg/mapper/entity_restapi/r2e	[no test files]
PASS
ok  	github.com/checkr/flagr/pkg/util	0.031s
CircleCI received exit code 0