Added initial CloudWatch Dashboard for RFS

[chelma]

Description

• Added a CloudWatch Dashboard for RFS that gets created when the RFS feature is enabled
• The Target Cluster section relies on having metrics provided by an Amazon OpenSearch Service domain to populate. Additionally, the user must supply the name of their domain in the Dashboard.
• One quirk is the mechanic we're using to represent how many "active" RFS Workers there are. We don't have a good way to represent the number of *desired* workers in a steady-state sense, so what we do is effectively report how many RFS Workers were alive in a given period by counting the number of separate instances of a CPU metric for our ECS containers. During steady-state operation, we'd expect the metric in that graph to resemble closely the user's desired worker count. However, if the workers are dying frequently (such as if there's no work), then new workers will be constantly popping up, reporting in, and dying. Each such worker adds a new entry to this sample count, and so the number skews higher than the desired number.

Issues Resolved

• https://opensearch.atlassian.net/browse/MIGRATIONS-2132

Testing

• Created the dashboard in my account and used to visualize a few simple migrations:

Check List

• [ ] New functionality includes testing
  • [ ] All tests pass, including unit test, integration test and doctest
• [ ] New functionality has been documented
• [X] Commits are signed per the DCO using --signoff

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.78%. Comparing base (892990a) to head (c9df41b). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1147 +/- ## ========================================= Coverage 80.78% 80.78% Complexity 2947 2947 ========================================= Files 399 399 Lines 15089 15089 Branches 1017 1017 ========================================= Hits 12190 12190 Misses 2288 2288 Partials 611 611 ``` | [Flag](https://app.codecov.io/gh/opensearch-project/opensearch-migrations/pull/1147/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/opensearch-project/opensearch-migrations/pull/1147/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `80.78% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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