fix(cli): requests get throttled when there is a large number of stacks

[otaviomacedo]

The CLI is making an unlimited number of concurrent GetTemplate requests. When there is a large number of stacks, CloudFormation may throttle the requests.

Limit concurrency to 20.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.95%. Comparing base (d1b4113) to head (96ddb11).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #927      +/-   ##
==========================================
+ Coverage   83.45%   83.95%   +0.49%     
==========================================
  Files          71       71              
  Lines       10401    10401              
  Branches     1317     1324       +7     
==========================================
+ Hits         8680     8732      +52     
+ Misses       1680     1628      -52     
  Partials       41       41              

Flag	Coverage Δ
suite.unit	83.95% <ø> (+0.49%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[rix0rrr]

Still seems like a high number?

Are the retry settings configured thoroughly enough for the CFN client?

[otaviomacedo]

This is the current retry strategy for the CFN client:

new ConfiguredRetryStrategy(11, (attempt: number) => 1000 * (2 ** attempt))

I ran an experiment with this strategy, against an account with 342 templates. I didn't start getting throttling errors until concurrency = 47. So, 20 might be playing it too safely, actually.

Just as a sanity check, without any retry strategy, even with concurrency = 2, I would sometimes get a throttling error.

[rix0rrr]

SGTM, thanks!