Adds support for reading server `private_key` from AWS Secrets Manager.
Combined with #31075, this should allow removing all common sensitive
secrets from the environment/config (if I missed any let me know). This
works with localstack for local development (set
`AWS_ENDPOINT_URL=$LOCALSTACK_URL`, `AWS_ACCESS_KEY_ID=test`, and
`AWS_SECRET_ACCESS_KEY=test`).
I did not include config options for `AWS_ACCESS_KEY_ID` and
`AWS_SECRET_ACCESS_KEY` because they are a bad practice vs role
credentials and defeat the purpose of this feature which is to remove
secrets from the environment/config.
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
<!-- Note that API documentation changes are now addressed by the
product design team. -->
- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.
- [x] Added/updated automated tests
- [x] Manual QA for all new/changed functionality
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Co-authored-by: Scott Gress <scott@fleetdm.com>
