Simplified and modularized IAM auth setup for MySQL connections.
<!-- Add the related story/sub-task/bug number, like Resolves#123, or
remove if NA -->
**Related issue:** Resolves#36846
Manually QA'ed by setting up RDS with IAM and running Fleet like:
```
FLEET_MYSQL_ADDRESS=fleet-iam-test-public.xxxxxxxxx.us-east-2.rds.amazonaws.com:3306 \
FLEET_MYSQL_USERNAME=fleet_iam \
FLEET_MYSQL_DATABASE=fleet \
FLEET_MYSQL_REGION=us-east-2 \
./build/fleet serve
```
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
- [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.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Refactor**
* Reorganized IAM authentication infrastructure for RDS databases to
improve code organization and maintainability.
* Enhanced the database connection layer to support flexible
authentication configuration methods while maintaining full backward
compatibility with existing configurations.
<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
