Add support for AWS shared credentials file

[brandond]

Proposed Changes

Add support for AWS shared credentials file

Also adds a CLI flag and fields for session token, which must be passed alongside the access key and secret when using temporary credentials. Ref: https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html#envvars-list-AWS_SESSION_TOKEN

Types of Changes

enhancement

Verification

Use shared credentials file (~/.aws/credentials) for S3:

1. Run aws configure as root
2. Enter access and secret key
3. Confirm that values are written to default profile in /root/.aws/credentials
4. Start K3s and save snapshots to S3 - you will need to specify the bucket and folder but NOT the access or secret key.

Testing

Linked Issues

• Support loading S3 credentials from shared credentials file #11609

User-Facing Change

etcd snapshot backup/restore now supports loading s3 credentials from an AWS SDK shared credentials file.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 47.66%. Comparing base (95700aa) to head (dc589a6).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/etcd/s3/s3.go	86.66%	1 Missing and 1 partial ⚠️
pkg/cli/server/server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11614      +/-   ##
==========================================
- Coverage   49.90%   47.66%   -2.24%     
==========================================
  Files         185      185              
  Lines       19344    19353       +9     
==========================================
- Hits         9654     9225     -429     
- Misses       8298     8795     +497     
+ Partials     1392     1333      -59     

Flag	Coverage Δ
e2etests	40.36% <0.00%> (-3.80%)	⬇️
inttests	35.04% <0.00%> (+0.03%)	⬆️
unittests	17.03% <88.00%> (+0.04%)	⬆️

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