feat(googlepolicy): add LogTransformPolicy (LogAdd) compiler and evaluator

[rafaelwestphal]

Summary

Implements LogTransformPolicy (LogAdd operation) end-to-end across protobuf definitions, pkg/googlepolicy, googlepolicyprocessor, googlecontrolplaneprovider, and the policy integration test suite:

• Protobuf (proto/policy/v1alpha1/log_transform_policy.proto): Defines LogTransformPolicy (id, repeated LogMatcher matches, repeated LogAdd add) and LogAdd (LogFieldSelector target, Value value, bool upsert) with generated Go bindings and wire/protojson round-trip tests.
• Shared Log Matcher & Attribute Setter Engine (pkg/googlepolicy/internal/matcher):
  • Extracts CompiledLogMatcher, CompileLogMatcher, and CompileLogExtractor into internal/matcher/log.go so logfilter and logtransform share log matching logic as peers without sibling imports.
  • Adds CompilePathSetter (supporting nested AttributePath map creation without leaving partial maps when upsert: false rejects an existing leaf or non-map ancestor) and CompileValueSetter.
• Compiler & Evaluator (pkg/googlepolicy/logtransform):
  • Implements googlepolicy.LogTransformPolicyEvaluator (TransformLog(ctx LogContext) TransformResult returning TransformNoMatch | TransformModified) for PolicyType = "log_transform".
  • Supports all valid LogFieldSelector targets (log_attribute, resource_attribute, scope_attribute, record_field [BODY, SEVERITY_TEXT, SEVERITY_NUMBER, TRACE_ID, SPAN_ID], and scope_field [NAME, VERSION; SCHEMA_URL is rejected at compile time]).
  • Includes unit tests (policy_test.go, 98.9% coverage) and fuzz tests (fuzz_test.go, FuzzLogTransformPolicy).
• Two-Stage Processor Pipeline (googlepolicyprocessor):
  • Evaluates Stage 1 (log_filter) before Stage 2 (log_transform) in deterministic PolicyName() order; dropped log records never enter Stage 2.
  • Records result="transformed" on processor_googlepolicy_records and processor_googlepolicy_policy_records (while preserving result="kept" on processor_googlepolicy_policy_records when a record is exempted by an ACTION_KEEP filter policy and subsequently transformed).
• Integration Tests (integration_test/policy_test):
  • Adds golden test testdata/log_transform_and_filter_ordering and TestLogFilterKeepAndTransformSelfMetrics.