Add helpers to obtain an aws-sdk-go-v2 client for programmatic access

[mjlshen]

What type of PR is this?

feature

What this PR does / Why we need it?

This allows other tools to get an AWS client and programatically interact with the AWS API. osdctl used to do this with the previous iteration of backplane-api and these helpers would assist with the effort in openshift/osdctl#357 instead of having to re-implement a lot of the parsing logic.

[wanghaoran1988]

/lgtm
/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mjlshen, wanghaoran1988

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[codecov-commenter]

Codecov Report

Merging #66 (520f293) into main (6e95b55) will increase coverage by 0.92%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
+ Coverage   43.90%   44.82%   +0.92%     
==========================================
  Files          37       37              
  Lines        2182     2224      +42     
==========================================
+ Hits          958      997      +39     
- Misses       1082     1083       +1     
- Partials      142      144       +2     

Impacted Files	Coverage Δ
cmd/ocm-backplane/cloud/credentials.go	66.15% <0.00%> (-12.03%)	⬇️

... and 3 files with indirect coverage changes

[openshift-ci]

@mjlshen: all tests passed!

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