Improvements to `resolve_secret_banner.ts`

[rtpascual]

From #845 (comment).

There are a couple enhancements that can be done for the resolve secret banner code snippet for customer lambdas:

• Multiline support for readability and inline comments (// some comment)
• Support for importing third party modules or other files

[edwardfoyle]

Done. There are still some possible improvements here (or different approaches we could take), but we will address those if / when future refactors of the ssm banner happen.