feat: Add option to define a delimiter for metric-alarms-by-multiple-dimensions

[caseycs]

Description

Add option to define a delimiter between alarm name and dimension key for metric-alarms-by-multiple-dimensions

Motivation and Context

While creating multiple alerts I noticed that there is no delimiter between common prefix and dimension resource name, so I had to prefix them by space character:

" ${local.project.name}-postgres-${local.env}"

and this does not look so pretty:

inputs = {
    items = {
        rds-cpu-usage = {
...
            dimensions = {
                " ${local.project.name}-postgres-${local.env}" = {
                   DBClusterIdentifier = "${local.project.name}-postgres-${local.env}" 
                }
            }
...
        }
...

Breaking Changes

None, default value falls back to previous behaviour.

How Has This Been Tested?

There are no examples that use metric-alarms-by-multiple-dimensions, so I tested it only on my local project.

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[antonbabenko]

This PR is included in version 5.3.0 🎉

[github-actions]

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