feat(infrastructure): Introduce shared infrastructure config package.

Add a new infrastructure package that centralises loading of the infrastructure config and instantiation of the secrets provider, replacing the per-package loading logic that previously lived in postgres.

infrastructure.DefaultConfig() lazily loads /etc/labkit/infrastructure.yaml and a file-backed secret provider rooted at /secrets/infrastructure, caching the result for subsequent callers.

Refactor the PostgreSQL client to consume *infrastructure.Config via a new WithInfrastructure option, which is primarily useful for testing. When no config is provided, postgres.New(ctx) falls back to DefaultConfig(), so the developer-friendly zero-argument API is preserved and LabKit will do the right thing.

As part of the refactor:

• Replace ErrUndefinedDependency with the more specific ErrNotConfigured and ErrMissingSecrets.
• Move secret lookups from a postgres-specific mount (/secrets/infrastructure/postgresql) to keyed lookups (postgresql/username, postgresql/password) against the shared provider rooted at /secrets/infrastructure.
• Rewrite postgres tests to construct *infrastructure.Config directly with a secret.MemoryProvider, avoiding filesystem fixtures and adding coverage for missing-secret error paths.

Issue: gitlab-com/gl-infra/platform/runway/team#884 (closed)