feat(config): support env substitution

[AkashKumar7902]

Summary

• expand environment variables in zot config string values using $VAR and ${VAR}
• apply the same substitution to referenced LDAP, OpenID/OAuth2, and session key secret files
• fail config loading when a referenced environment variable is unset
• document the syntax in examples/README.md

Fixes #2521

Validation

• GOEXPERIMENT=jsonv2 go test ./pkg/cli/server -count=1
• git diff --check