config: apply environment overrides before unmarshal - #902

Fixes #833

Nested free-form collector configuration supplied through GNMIC_*
environment variables was not reflected in the unmarshaled Config.

Apply environment-backed free-form configuration before unmarshaling,
while limiting underscore-based nesting to sections whose keys are not
known ahead of time:

• clustering
• outputs
• inputs
• processors
• loader
• actions

Known struct-backed configuration such as cluster-name, log-file,
and format continues to use Viper's AutomaticEnv and environment
key replacer. This avoids treating underscores in variables such as
GNMIC_LOG_FILE as nesting separators while still supporting nested
values such as GNMIC_CLUSTERING_LOCKER_ADDRESS.

Regression coverage verifies:

• known struct-backed environment variables remain handled by Viper
• GNMIC_LOG_FILE and GNMIC_CLUSTER_NAME override their fields
• GNMIC_CLUSTERING_LOCKER_ADDRESS overrides nested free-form config
• unrelated clustering values from the configuration file are preserved
• underscore splitting is restricted to the six free-form sections

Testing:

• focused environment/config regression tests
• go test ./pkg/config -count=1
• go test ./... -count=1
• go vet ./...
• git diff --check