Adds `PrefixProvider`, a thin `Provider` decorator that prepends a
fixed string to the secret name before delegating to an underlying
`Provider`. The prefix is applied verbatim, so callers that want path
semantics must include a trailing slash (e.g. `"postgresql/"`).

feat(secret): Add `PrefixProvider` to prepend a path prefix to secret names.

See merge request !554

Merged-by: Luke Hollinda <lhollinda@gitlab.com>
Approved-by: Luke Hollinda <lhollinda@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Florian Forster <fforster@gitlab.com>