Adds `ProjectionProvider`, a new `Provider` implementation that resolves secret
paths by reading field values from a protobuf message rather than hardcoding
key names. `NewProjectionProvider` wraps an underlying `Provider` with a
`proto.Message` whose string fields map logical credential names (e.g.
`"username"`, `"password"`) to the actual key names stored in the Secret.

`Get` looks up the requested name against the message descriptor by text name,
rejects non-string fields with `ErrProjectionInvalid`, and returns
`ErrProjectionMissing` when an optional field is absent — distinguishing
internal logic errors from user configuration errors so callers can surface the
right message.

Adds `TestProjection` covering successful lookups against
`PostgreSQLProjection` and the missing-field path, and
`TestProjection_internalErrors` covering a non-string field (`port`) and a
field name that does not exist in the descriptor.

Merges `TestProjectionProvider` and `TestProjectionProvider_internalErrors`
into a single table-driven test, covering successful lookups, unset and empty
fields (`ErrProjectionMissing`), a non-existent field name, and a non-string
field type (both `ErrProjectionInvalid`). Adds `t.Parallel()` at both the
top-level and sub-test level.

Renames the `secretProvider` field and constructor parameter to `base`, and the
`Get` parameter from `secretName` to `fieldName`. The old name was misleading:
the argument is a field in the projection proto, not a secret name. The local
holding the resolved value is renamed to `secretName`, which is what it
actually is.

Documents that `fieldName` is matched against the projection proto's field JSON
names (camelCase) since users configure these via YAML, and links the
`ErrProjectionMissing` and `ErrProjectionInvalid` sentinels in the doc comment.

feat(secret): Add proto-driven `ProjectionProvider` provider for secret key mapping.

See merge request !553

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