Fix cleanup process for plugin

[goccy]

If an error has already been returned from instanceModErrCh, calling write() will hang. Therefore, if an error is returned even once, the closed flag will be set to true, and any subsequent write() calls will immediately return.

https://github.com/mercari/grpc-federation/compare/fix-plugin?expand=1#diff-3b3c6f33ba272faf69f2b02309c3198a65dd408af919c76064348877a20e2961R209-R211

Also, since performing the plugin initialization process in every NewLocalValue caused performance issues, it will be restricted to cases where the context library is used.

Finally, I improved the process of creating the enum list.

[github-actions[bot]]

Code Metrics Report

	main (5268f6c)	#207 (3c580e6)	+/-
Coverage	65.0%	65.0%	+0.0%
Code to Test Ratio	1:0.3	1:0.3	-0.0
Test Execution Time	7m3s	7m9s	+6s

Details

``` diff | | main (5268f6c) | #207 (3c580e6) | +/- | |---------------------|----------------|----------------|-------| + | Coverage | 65.0% | 65.0% | +0.0% | | Files | 69 | 69 | 0 | | Lines | 11475 | 11574 | +99 | + | Covered | 7457 | 7522 | +65 | - | Code to Test Ratio | 1:0.3 | 1:0.3 | -0.0 | | Code | 36583 | 36758 | +175 | | Test | 12567 | 12567 | 0 | - | Test Execution Time | 7m3s | 7m9s | +6s | ```

Code coverage of files in pull request scope (71.2% → 71.0%)

Files	Coverage	+/-
generator/code_generator.go	79.6%	-0.0%
grpc/federation/cel.go	0.0%	0.0%
grpc/federation/cel/plugin.go	0.0%	0.0%
resolver/def.go	87.0%	-7.1%
resolver/message.go	75.3%	-0.0%
resolver/resolver.go	81.1%	+0.1%
resolver/value.go	77.8%	-1.0%

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Reported by octocov