Add optional field to CopyOperation

[sohel2020]

Summary

Closes #368

Adds a per-copy-entry optional flag to CopyOperation so users can control whether a copy operation that matches no files fails or silently skips.

• Added Optional bool field to CopyOperation in api/v1beta1/artifactgenerator_types.go
• Builder respects the flag at three error sites: glob no-match, all-files-excluded, and non-glob path-not-found
• Regenerated CRD manifests via make generate && make manifests
• Updated spec docs in docs/spec/v1beta1/artifactgenerators.md
• Added three test cases covering each skip path

Behavior

Scenario	optional: false (default)	optional: true
Glob matches no files	Error	Silent skip
All matched files excluded	Error	Silent skip
Non-glob source path missing	Error	Silent skip
Missing source alias	Error (always)	Error (always)

Backward compatible: Optional is bool with zero value false. Existing CRs without this field preserve current error behavior. The CRD change is additive.

Changes

• api/v1beta1/artifactgenerator_types.go - new Optional field on CopyOperation
• config/crd/bases/source.extensions.fluxcd.io_artifactgenerators.yaml - regenerated CRD
• internal/builder/builder.go - check op.Optional before returning no-match errors
• internal/builder/builder_test.go - three new test cases in TestBuildErrors
• docs/spec/v1beta1/artifactgenerators.md - documented optional in Copy Operations section

Test plan

• go test ./internal/builder/ -run TestBuild -count=1 passes (all existing + new tests)
• go vet ./... clean
• go fmt ./... clean
• make test (full suite, requires network for CRD fetch)