support existing secrets for aws creds

[sh4sh]

What

Partially addresses airbytehq/airbyte#22529
There are helm values to override AWS_SECRET_ACCESS_KEY and AWS_ACCESS_KEY_ID with existing secrets, but at the moment they don't seem to work. I added some logic to support these values.

Can this PR be safely reverted / rolled back?

If you know that your PR is backwards-compatible and can be simply reverted or rolled back, check the YES box.

Otherwise if your PR has a breaking change, like a database migration for example, check the NO box.

If unsure, leave it blank.

• YES 💚
• NO ❌

[github-actions]

Airbyte Code Coverage

There is no coverage information present for the Files changed

Total Project Coverage	27.67%	🍏

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

Your branch is not currently up-to-date with main. Please update your branch before attempting to snapshot your PR.