Lower the concurrency when "TOOMANYREQUESTS: Rate exceeded" occurs

**Describe the problem/challenge you have**
When talking to some registries imgpkg might be doing more requests than the registry allows.

**Describe the solution you'd like**
Whenever imgpkg encounters the TOOMANYREQUESTS error it would be interesting to automatically adapt by lowering the number of concurrent workers it is using.

**Anything else you would like to add:**
Investigate to understand what it would look like to implement this feature.

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