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.

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.

Also bumps image-automation-controller/api to a pseudo-version that
exposes SigningKey.Type and the SigningKeyTypeGPG/SigningKeyTypeSSH
constants; the bump is bundled here so the rest of the branch builds
incrementally. Refs fluxcd/pkg#398[1].

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

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

go.sum

@@ -178,8 +178,8 @@ github.com/fluxcd/go-git-providers v0.26.0 h1:0DUsXc1nS9Fe4n8tXSEUCGemWzHShd66gm
 github.com/fluxcd/go-git-providers v0.26.0/go.mod h1:VJDKUOhZwNAIqDF5iPtIpTr/annsDbKMkPpWiDMBdpo=
 github.com/fluxcd/helm-controller/api v1.5.5 h1:xQA/9gbifMvZPGhSNKHsrkq829dI/yTBASVdYp9/s4Y=
 github.com/fluxcd/helm-controller/api v1.5.5/go.mod h1:lTgeUmtVYExMKp7mRDncsr4JwHTz3LFtLjRJZeR98lI=
-github.com/fluxcd/image-automation-controller/api v1.1.4 h1:i78AwbcICXSX+a1MQwjNA1Uxxs1e3kfi3EJ21fWzb7w=
-github.com/fluxcd/image-automation-controller/api v1.1.4/go.mod h1:lkD/drkD6Wc+2SDjVj5KqfozEucTLFexWgby/5ft660=
+github.com/fluxcd/image-automation-controller/api v1.0.1-0.20260529125431-20ebc65ab20f h1:pjHh/w2xRd9u20J0c8H0EEoPAurJndm2XNF+/mem3EE=
+github.com/fluxcd/image-automation-controller/api v1.0.1-0.20260529125431-20ebc65ab20f/go.mod h1:XNWgNSF7GVZgGx6qTI+8jhiH0S+a/hNtNvBwPcxOotw=
 github.com/fluxcd/image-reflector-controller/api v1.1.2 h1:VPwUgA8WyPVVs16uSkwvjOAY6pvTYgAb0fL90t0RKLE=
 github.com/fluxcd/image-reflector-controller/api v1.1.2/go.mod h1:j4JSIocL42HQ77Veg1t60sApOy+lng8/cbXHXGSnfi0=
 github.com/fluxcd/kustomize-controller/api v1.8.5 h1:4fGPh6foGVKUUbt5OjVzbC5iTyX+Q+NS50atPboDC4w=
@@ -202,8 +202,8 @@ github.com/fluxcd/pkg/chartutil v1.26.0 h1:bgXwDKl2uGITSDxOXe4N5HTXAL0ilk4YVbPLU
 github.com/fluxcd/pkg/chartutil v1.26.0/go.mod h1:sWDcF//xpIwZ/MQupwTmuM/SgGCkfsTd9BVOQJ3cTjM=
 github.com/fluxcd/pkg/envsubst v1.7.0 h1:PL9Nj/V2fgaMR9KYZR7mEEw+vlYgP80nFZjOQQKAfJs=
 github.com/fluxcd/pkg/envsubst v1.7.0/go.mod h1:aoWeSIOamhqBZ3bHVj1GDwpdA10DXrI8yYbyjPiFly0=
-github.com/fluxcd/pkg/git v0.50.0 h1:gbGmSTjQ6CxqFmT9ZkLfYh8kG7CHqc7gHoPqcygixK8=
-github.com/fluxcd/pkg/git v0.50.0/go.mod h1:OgaHoS0iR0GuLl+f778X7NrGy1pDH7xcpF/nsCRgJ9g=
+github.com/fluxcd/pkg/git v0.52.0 h1:dgsliHdaLADUcDO4pI0pc11N4dZ21NfDdhNcgRNuAkM=
+github.com/fluxcd/pkg/git v0.52.0/go.mod h1:mOvFDxoiuz+Mm4Ux1wKeTTckvBgZFvbTK8lNxmVHzKs=
 github.com/fluxcd/pkg/gittestserver v0.29.0 h1:2j03zKVL6iVn6oiUuecG/O/3Q1pULWM9JrF/HSjkpnc=
 github.com/fluxcd/pkg/gittestserver v0.29.0/go.mod h1:O8151jV0ppBZTb9IUXMjxh6hZpkiuLq8JQHDBPOkZFw=
 github.com/fluxcd/pkg/kustomize v1.32.0 h1:5lLT2dgR+JrcoJHB7/K50o0AcJikKvXcRd3r7jIYZC8=
@@ -220,8 +220,8 @@ github.com/fluxcd/pkg/ssh v0.25.0 h1:4Y9WmuNqyKvH759UznU5DGHRcOuoJ/dQM6sbsaDZYYM
 github.com/fluxcd/pkg/ssh v0.25.0/go.mod h1:Fli2Ogu4uaIVGbCy+r0vvZlMO0RfuInyNY1q2FVIx0o=
 github.com/fluxcd/pkg/tar v1.2.0 h1:T6WFB5M0YRHktlrgdKNskqpdp76TVDdWTOeuWz33CFs=
 github.com/fluxcd/pkg/tar v1.2.0/go.mod h1:Wlalp5vIVe+BbckkKkqExKcoHAeeWJPAzwK7ONeFcS0=
-github.com/fluxcd/pkg/version v0.15.0 h1:E2Ju4i0vj8ZXLHKz/F4a8JTmDh7Jcg8okB0hK5rEoTM=
-github.com/fluxcd/pkg/version v0.15.0/go.mod h1:LEHnvLMgbTk4kelF+JHHzaG77kY9uTWodMtadPRMEW8=
+github.com/fluxcd/pkg/version v0.16.0 h1:VR9+143LAwbyUSAaMhiJHbfsiU+fTjA9L/3dr1ucfrI=
+github.com/fluxcd/pkg/version v0.16.0/go.mod h1:2M/l90CmbDaD21JTh77hjwaUbd/YM96+Fo8x4fMdxLI=
 github.com/fluxcd/source-controller/api v1.8.5 h1:mLKc9YVMk46JCt1BQbkG6irkrpBZp95kiXh2+GYB6KQ=
 github.com/fluxcd/source-controller/api v1.8.5/go.mod h1:sio4t49RDx+S1etHRFAEEw8qfVuw0KKlOg8bRVlEYPM=
 github.com/fluxcd/source-watcher/api/v2 v2.1.1 h1:1LfT50ty+78MKKbschAZl28QbVqIyjaNq17KmW5wPJI=