Add ssh credentials for git

[eddymoulton]

Adds support for passing through ssh git credentials down to LibGit2Sharp

Will update the LibGit2Sharp version to a real release before merging.

Server side change: https://github.com/OctopusDeploy/OctopusDeploy/pull/42731

Closes MD-1680

[eddymoulton]

With shared contracts, I figure a new Octopus server might be talking to an old calamari?

Unsure if this is truely needed to be nullable

[gitguardian]

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