Allow digits only project key for SonarCloud

[sebastien-marichal]

See this internal post for more context.

SonarCloud allows project keys with only digits such as 1989, when invoking S4NET with such a key, it will produce the following error:

Invalid project key. Allowed characters are alphanumeric, '-', '_', '.' and ':', with at least one non-digit.

[megan-wilson-sonarsource]

I've updated the Discuss post. Please review.

[sebastien-marichal]

This is under analysis.
The policy might need to be enforced in SonarCloud.
Waiting for feedback.