Migrate bootstrap signing to generic Signer

Bumps fluxcd/pkg/git to a pseudo-version exposing 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

@@ -180,8 +180,8 @@ 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=
@@ -204,8 +204,8 @@ github.com/fluxcd/pkg/chartutil v1.24.0 h1:Rh9o50eJUnAioL5q2JVPxO4DjFjwHwPE5K9Lc
 github.com/fluxcd/pkg/chartutil v1.24.0/go.mod h1:oycULCP00m46dxiskme1Yawe74UFLZzX0jqHb6xzdmQ=
 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 v0.49.0 h1:OtI0TjMVC/GV/yPos4waA5fAs5u3/2YEYBFuZuJA3Rc=
-github.com/fluxcd/pkg/git v0.49.0/go.mod h1:Xnrqtz60/0jqfcy5UsY9sxlvPmlvzelPsej2v3L7wW8=
+github.com/fluxcd/pkg/git v0.49.1-0.20260529122759-f46ad90373c5 h1:ZPpsEw33U/2JmgeGIbgdAN0UX+GM01sE+cb1OuuSQwY=
+github.com/fluxcd/pkg/git v0.49.1-0.20260529122759-f46ad90373c5/go.mod h1:OgaHoS0iR0GuLl+f778X7NrGy1pDH7xcpF/nsCRgJ9g=
 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=
@@ -352,6 +352,8 @@ github.com/hashicorp/golang-lru/arc/v2 v2.0.5 h1:l2zaLDubNhW4XO3LnliVj0GXO3+/CGN
 github.com/hashicorp/golang-lru/arc/v2 v2.0.5/go.mod h1:ny6zBSQZi2JxIeYcv7kt2sH2PXJtirBN7RDhRpxPkxU=
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+github.com/hiddeco/sshsig v0.2.0 h1:gMWllgKCITXdydVkDL+Zro0PU96QI55LwUwebSwNTSw=
+github.com/hiddeco/sshsig v0.2.0/go.mod h1:nJc98aGgiH6Yql2doqH4CTBVHexQA40Q+hMMLHP4EqE=
 github.com/homeport/dyff v1.10.2 h1:XyB+D0KVwjbUFTZYIkvPtsImwkfh+ObH2CEdEHTqdr4=
 github.com/homeport/dyff v1.10.2/go.mod h1:0kIjL/JOGaXigzrLY6kcl5esSStbAa99r6GzEvr7lrs=
 github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=