switch to ex_aws library

[panentheos]

Replaces aws-elixir library with ex_aws plus a module to support Polly. The aws-elixir library has no built-in mechanism for querying ECS instance metadata, so it doesn't work when deployed. This should be a no-op, functionality wise.

[github-actions]

Coverage of commit 25a0148

Summary coverage rate:
  lines......: 6.1% (2 of 33 lines)
  functions..: 33.3% (2 of 6 functions)
  branches...: no data found

Files changed coverage rate:
                          |Lines       |Functions  |Branches    
  Filename                |Rate     Num|Rate    Num|Rate     Num
  ==============================================================
  lib/web_api.ex          | 3.1%     32|20.0%     5|    -      0

Download coverage report

[digitalcora]

Looks good, I had one minor question but it should not be blocking!

[digitalcora]

Curious about the choice of "stub" value here. Can it be ""? Is there something special about the space character? (or rather, <<32>>, since this data surely wouldn't be interpreted as UTF-8 text)

[panentheos]

I tried "" originally, and it caused the API to respond differently in a way that complicated the code (I think it omitted the body entirely, instead of returning an empty string). I didn't re-check that with the new library, but it seems like using a non-empty stub is a cheap way to prevent similar downstream edge cases. There's nothing special about the choice of space as the character.