Enable building GHES specific clients

[thorrsson]

Adds arguments to allow the caller to specify building for GHES as well as which version of the API spec to build from.

ex
./script/generate-go.sh ghes 3.12

Passing ghes without a version will result in:

@thorrsson ➜ /workspaces/source-generator (enable-ghes) $ ./scripts/generate-go.sh ghes 
microsoft.openapi.kiota is installed. Updating to sync with the required version
Tool 'microsoft.openapi.kiota' was reinstalled with the prerelease version (version '1.13.0-preview.202403280001').
+ [ -z ghes ]
+ [ ghes = ghes ]
+ [ -z  ]
+ echo generating for GHES requires that you specify a version as the second argument. ex ./scripts/generate-go.sh ghes 3.12
generating for GHES requires that you specify a version as the second argument. ex ./scripts/generate-go.sh ghes 3.12
+ exit 1