Add etcd s3 config secret implementation

[brandond]

Proposed Changes

Add implementation of etcd snapshot s3 config secret support, as covered by the ADR from #9364

• Move snapshot structs and functions into pkg/etcd/snapshot
• Move s3 client code and functions into pkg/etcd/s3
• Refactor pkg/etcd to track snapshot and s3 moves
• Add support for reading s3 client config from secret
• Add minio client cache, since S3 client configuration can now be changed at runtime by modifying the secret, and don't want to have to create a new minio client every time we read config.

Types of Changes

New feature

Verification

See ADR for intended use

Testing

Yes, S3 integration test is updated

Linked Issues

• Add etcd snapshot s3 config secret support #10370
• Add ADR for support for etcd s3 config secret #9364

User-Facing Change

A proxy can now be configured for use when uploading etcd snapshots to a s3-compatible storage service. This overrides any proxy settings passed via environment variables.
Credentials and endpoint configuration for storing etcd snapshots on a s3-compatible storage service can now be read from a Secret, instead of passing them via the CLI or config file. See https://github.com/k3s-io/k3s/blob/master/docs/adrs/etcd-s3-secret.md for more information.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 48.58841% with 346 lines in your changes missing coverage. Please review.

Project coverage is 43.56%. Comparing base (b4d4ed8) to head (270d540).

Files	Patch %	Lines
pkg/etcd/snapshot/types.go	6.55%	114 Missing ⚠️
pkg/etcd/snapshot.go	4.08%	94 Missing ⚠️
pkg/etcd/s3/s3.go	80.58%	39 Missing and 21 partials ⚠️
pkg/etcd/etcd.go	13.79%	23 Missing and 2 partials ⚠️
pkg/etcd/snapshot_handler.go	0.00%	22 Missing ⚠️
pkg/cli/server/server.go	0.00%	13 Missing and 1 partial ⚠️
pkg/etcd/snapshot_controller.go	0.00%	10 Missing ⚠️
pkg/etcd/s3/config_secret.go	87.50%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10340      +/-   ##
==========================================
- Coverage   47.97%   43.56%   -4.42%     
==========================================
  Files         177      179       +2     
  Lines       14807    14892      +85     
==========================================
- Hits         7104     6487     -617     
- Misses       6363     7207     +844     
+ Partials     1340     1198     -142     

Flag	Coverage Δ
e2etests	36.35% <2.37%> (-10.19%)	⬇️
inttests	36.66% <0.59%> (-0.40%)	⬇️
unittests	13.39% <46.50%> (+2.03%)	⬆️

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