libgit2: Disable connection caching

[pjbgf]

Connection caching was a feature created to resolve upstream issues raised from concurrent ssh connections.
Some scenarios were based on multiple key exchange operations happening at the same time.

This PR removes the connection caching, and instead:

• Services Session.StdoutPipe() as soon as possible, as it is a known source of blocking SSH connections.
• Reuse SSH connection within the same subtransport, eliminating the need for new handshakes when talking
  with the same server.
• Simplifies the entire transport logic for better maintainability.

This also fixes the panic below when connecting to AWS CodeCommit with Managed Transport.
Note that this logic is not captured by the panic recovery due to its execution path not starting from the Checkout func.
Additional changes are being made to completely remove the dependency to SmartCertificateCheck, which will ensure this does not happen at all going forwards. But that will be dealt with in subsequent PRs.

panic: invalid pointer handle: 0x7fd35c005e80

goroutine 470 [running]:
github.com/libgit2/git2go/v33.(*HandleList).Get(0xab1c5ed5923f82a4?, 0x7fd35c005e80)
        /home/levi/go/pkg/mod/github.com/libgit2/git2go/[email protected]/handles.go:64 +0x12e
github.com/libgit2/git2go/v33.certificateCheckCallback(0x240ca1cc0fc19dc6?, 0xc000a17080, 0x1, 0x4a7484aa2de92c6f?, 0x76f988da5cb0a9dc?)
        /home/levi/go/pkg/mod/github.com/libgit2/git2go/[email protected]/remote.go:445 +0x3d
github.com/libgit2/git2go/v33._Cfunc_git_transport_smart_certificate_check(0x7fd35c0085f0, 0xc000a17080, 0x1, 0x7fd3a0001050)
        _cgo_gotypes.go:8790 +0x4c
github.com/libgit2/git2go/v33.(*Transport).SmartCertificateCheck.func9(0x7fd3a0001050?, 0x29?, 0x90?, 0x86062dda3eacc8fb?)
        /home/levi/go/pkg/mod/github.com/libgit2/git2go/[email protected]/transport.go:170 +0x67
github.com/libgit2/git2go/v33.(*Transport).SmartCertificateCheck(0xe28033dd618b378?, 0xc000efdcb0, 0x1, {0xc000e4df50, 0x29})
        /home/levi/go/pkg/mod/github.com/libgit2/git2go/[email protected]/transport.go:170 +0x325
github.com/fluxcd/source-controller/pkg/git/libgit2/managed.(*sshSmartSubtransport).Action.func1({0xc000e4df50, 0x29}, {0x306c073?, 0x7?}, {0x33cd170, 0xc000d91380})
        /home/levi/go/src/github.com/fluxcd/source-controller/pkg/git/libgit2/managed/ssh.go:208 +0x1bc
golang.org/x/crypto/ssh.(*handshakeTransport).client(0xc000b806e0, {0x33c54f8, 0xc000479960}, 0x1b?)
        /home/levi/go/pkg/mod/golang.org/x/[email protected]/ssh/handshake.go:698 +0x113
golang.org/x/crypto/ssh.(*handshakeTransport).enterKeyExchange(0xc000b806e0, {0xc000d76fc0, 0x101, 0x101})
        /home/levi/go/pkg/mod/golang.org/x/[email protected]/ssh/handshake.go:611 +0x3f4
golang.org/x/crypto/ssh.(*handshakeTransport).kexLoop(0xc000b806e0)
        /home/levi/go/pkg/mod/golang.org/x/[email protected]/ssh/handshake.go:301 +0xa6
created by golang.org/x/crypto/ssh.newClientTransport
        /home/levi/go/pkg/mod/golang.org/x/[email protected]/ssh/handshake.go:135 +0x236

[pjbgf]

Test images based on version 830771f:

• source-controller: quay.io/paulinhu/source-controller:v0.24.4-cacheless@sha256:61930cad1da900f209b396f20c2f7740ff32b5cf1bb4ab7892200790c00a5f4b
• image-automation-controller: quay.io/paulinhu/image-automation-controller:v0.22.2-cacheless@sha256:87823667cfc4c6e395d996ceaee92a1b5059a8950884f2c3aa49488dcbed81f5

UPDATE: changed IAC image to v0.22.2-cacheless

[pjbgf]

New image for source controller based on 6d51758:
ghcr.io/fluxcd/source-controller:rc-6d517589

UPDATE: replace image with official release candidate version.

[pjbgf]

Removed "enable managed transport" commit from this PR, we shall treat it as a separate PR.

[hiddeco]

Besides the nitpick, this looks good to me. Thanks @pjbgf 🙇

[aryan9600]

just one nitpick, otherwise it LGTM! 🚀