As a:
spk user
I want:
to be able to reference an existing variable group
So that:
multiple application repositories can share a single variable group in an AzDo project
Describe the solution you'd like:
spk project add-variable-group <variable-group-name> command that is similar to spk project create-variable-group <variable-group-name>, but instead of creating a variable group, it may check for the existence of the variable group in azdo, then it adds the reference to the project's bedrock.yaml AND it adds the variable group for all linked service build pipelines.

Acceptance Criteria:

Describe alternatives you've considered:
We could overload spk project add-variable-group <variable-group-name> and check the variable group exists already. If it does, log a warning then skip creating the VG in azdo, but continue to add it to bedrock.yaml and the related pipelines.

Additional context:

Does this require updates to documentation?:
yes