Get credentials from a process as specified in AWS config

[davidkretch]

Get credentials from a process as specified in the AWS config file's credential_process item. See https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-sourcing-external.html.

Addresses a part of #253.

[codecov]

Codecov Report

Merging #328 into main will decrease coverage by 0.82%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
- Coverage   86.28%   85.46%   -0.83%     
==========================================
  Files          52       52              
  Lines        2800     2827      +27     
==========================================
  Hits         2416     2416              
- Misses        384      411      +27     

Impacted Files	Coverage Δ
paws.common/R/credential_providers.R	9.90% <0.00%> (-3.19%)	⬇️
paws.common/R/credentials.R	100.00% <ø> (ø)

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