Support flatten function

[goccy]

Also update cel-go to latest

[github-actions]

Code Metrics Report

	main (c0aa252)	#264 (245c2ba)	+/-
Coverage	66.2%	66.1%	-0.1%
Code to Test Ratio	1:0.4	1:0.4	-0.0
Test Execution Time	8m6s	7m25s	-41s

Details

  |                     | main (c0aa252) | #264 (245c2ba) |  +/-  |
  |---------------------|----------------|----------------|-------|
- | Coverage            |          66.2% |          66.1% | -0.1% |
  |   Files             |             77 |             77 |     0 |
  |   Lines             |          13161 |          13183 |   +22 |
  |   Covered           |           8716 |           8716 |     0 |
- | Code to Test Ratio  |          1:0.4 |          1:0.4 |  -0.0 |
  |   Code              |          40741 |          40779 |   +38 |
+ |   Test              |          15732 |          15734 |    +2 |
+ | Test Execution Time |           8m6s |          7m25s |  -41s |

Code coverage of files in pull request scope (90.8% → 70.4%)

Files	Coverage	+/-
grpc/federation/cel/list.go	70.4%	-20.4%

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Reported by octocov