Add buffercache

[arl]

❓ What

Add BufferCache and SplitWriter reusable components in pkg.
Move awsutils into ./pkg

🔨 How to test

1. List all steps necessary;
2. To test this pull request.

✅ Checklists

This section contains a list of checklists for common uses, please delete the checklists that are useless for your current use case (or add another checklist if your use case isn't covered yet).

• Is there unit/integration test coverage for all new and/or changed functionality added in this PR?
• Have the changes in this PR been functionally tested?
• Has make gofmt-write been run on the code?
• Has make govet been run on the code? Has the code been fixed accordingly to the output?
• Have the changes been added to the CHANGELOG.md file?
• Have the steps in CONTRIBUTING.md been followed to update a Go module?

[tommyblue]

lgtm, what do you think of adding a paragraph in the readme to explain the existence of those pkgs and how/why to use them?

[arl]

lgtm, what do you think of adding a paragraph in the readme to explain the existence of those pkgs and how/why to use them?

I can state the existence of pkg in the readme , though I'm not sure about the utility of going into further details there, these packages have good name and documentation which makes them pretty self-explanatory.