Rework the DSN construction in `clientOptions.newFromProto` to support
secrets produced by a variety of PostgreSQL operators. Presets are
shipped for CNPG, Crunchy Data, and Zalando; users can also supply a
custom field projection via `PostgreSQLSecret.Projection`.

Each DSN field is resolved either from inline config (`Host`, `Port`,
`DatabaseName`, `Username`) or from a secret field looked up via the
projection. `Password` is always resolved from a secret. Errors across
all fields are joined so callers see every misconfiguration at once,
not just the first.

As defense in depth, `resolveSecret` rejects secret references
containing a slash to prevent path traversal into other secret
namespaces. The schema already constrains secret names to valid
Kubernetes resource names.

Depends on the matching Fairway schema change in
gitlab-com/gl-infra/platform/runway/fairway!63, which introduces
`PostgreSQLSecret`, `PostgreSQLFormat`, and `PostgreSQLProjection` and
turns `PostgreSQL.Host` into a `*string`. The `go.mod` entry points at
a pre-release commit until that MR is merged and tagged.

Issue: gitlab-com/gl-infra/platform/runway/team#873

feat(postgres): Support multiple PostgreSQL operator secret formats.

See merge request !484

Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Co-authored-by: Florian Forster <fforster@gitlab.com>