SBE needs git-info for individual repo submodules

Related to https://github.com/dotnet/source-build/issues/3565#issuecomment-1656144736

The [SBE repo](https://github.com/dotnet/source-build-externals) has a git submodule for [azure-activedirectory-identitymodel-extensions-for-dotnet](https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet). The script which dynamically injects assembly version info needs to be reimplemented as a Bash script. But the implementation relies on [git to get the current commit SHA](https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/620be62dda649d4c63e88ec9ae994a12cf54c707/updateAssemblyInfo.ps1#L38). This won't work when the .NET source is extracted from a tarball to be built, outside the context of a git repo configuration.

Instead, we need the same concept that the VMR has today: [git-info](https://github.com/dotnet/dotnet/tree/main/prereqs/git-info). But this needs to be defined within the SBE repo itself, scoped to its own submodules. This will allow scripts or other logic to consume that state rather than relying on git. It specifically should be contained within the SBE repo to allow the logic which needs that info to work when building the repo locally or in CI.