For #31048
This change includes some refactoring of orbit code. No functional
changes. Moved non-Linux-specific code from `securehw_linux.go` to
`securehw_tpm.go` so that tests on any platform can use it.
There are no server changes impacting the upcoming 4.72 release. Just
tests.
# Checklist for submitter
## Testing
- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually
## fleetd/orbit/Fleet Desktop
- [x] If the change applies to only one platform, confirmed that
`runtime.GOOS` is used as needed to isolate changes
- [x] Verified that fleetd runs on macOS, Linux and Windows
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Introduced a new TPM 2.0-based secure hardware interface, enabling
creation, loading, and management of ECC keys within a TPM device.
* Added support for both standard and RFC 9421-compatible HTTP
signatures using TPM-backed keys.
* **Bug Fixes**
* Improved error handling and resource management for TPM operations.
* **Tests**
* Added comprehensive unit tests for TPM key file loading scenarios.
* Introduced integration tests using a simulated TPM device to validate
end-to-end secure hardware and SCEP workflows.
* **Chores**
* Updated dependencies for enhanced compatibility and security.
* Modernized build constraints for improved maintainability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
