Migrate bootstrap signing to generic Signer

Bumps fluxcd/pkg/git to v0.52.0, which exposes the generic
signature.Signer interface and the NewOpenPGPSigner / NewSSHSigner
constructors, and migrates pkg/bootstrap's two WithSigner call sites
accordingly. Refs fluxcd/pkg#398[1].

Adds a parallel WithSSHCommitSigning option alongside the existing
WithGitCommitSigning so callers can sign commits with an SSH private
key. PlainGitBootstrapper now dispatches through a new resolveSigner
helper that returns either an OpenPGP or SSH signer; the
repository.WithSigner option is appended conditionally to avoid the
typed-nil interface hazard the new generic field introduces.

The bootstrap path's OpenPGP entity selector is renamed and exported
as SelectOpenPGPSigningEntity so the flux CLI's pre-flight (introduced
later in this branch) can call it directly instead of carrying a
duplicate.

[1]: https://github.com/fluxcd/pkg/issues/398

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

go.mod

@@ -23,7 +23,7 @@ require (
 	github.com/fluxcd/pkg/auth v0.51.0
 	github.com/fluxcd/pkg/chartutil v1.26.0
 	github.com/fluxcd/pkg/envsubst v1.7.0
-	github.com/fluxcd/pkg/git v0.50.0
+	github.com/fluxcd/pkg/git v0.52.0
 	github.com/fluxcd/pkg/kustomize v1.35.0
 	github.com/fluxcd/pkg/oci v0.66.0
 	github.com/fluxcd/pkg/runtime v0.108.0
@@ -31,7 +31,7 @@ require (
 	github.com/fluxcd/pkg/ssa v0.75.0
 	github.com/fluxcd/pkg/ssh v0.25.0
 	github.com/fluxcd/pkg/tar v1.2.0
-	github.com/fluxcd/pkg/version v0.15.0
+	github.com/fluxcd/pkg/version v0.16.0
 	github.com/fluxcd/source-controller/api v1.9.0
 	github.com/fluxcd/source-watcher/api/v2 v2.1.1
 	github.com/go-git/go-git/v5 v5.19.1