feat: use custom namespace rather than stack-name

[abdul-ashour]

What did you implement:

Allow using custom namespace value rather than the stack-name for alerts configured with patterns.
Closes #142

How did you implement it:

The implementation simply considers namespace value specified in the alert. The changes do not introduce any additional option

How can we verify it:

You just need to add namespace to your alert definition then deploy to AWS. You should see your metric created under the specified namespace.

[et304383]

As the author of #142 I'm curious what we can do to move this PR along.