Fixes#30475
# Checklist for submitter
- [x] Manual QA for all new/changed functionality
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Introduced support for HTTP message signing in agent-server
communications, enhancing request authentication.
* Added a configurable option to control the probability of agents using
HTTP message signatures via a new command-line flag.
* **Bug Fixes**
* Improved error logging for issues encountered during HTTP signature
key retrieval, providing better visibility into failures.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Fixes#30473
# Checklist for submitter
- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
- [x] Added/updated automated tests
- [ ] Manual QA for all new/changed functionality
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added support for TPM-backed host identity certificates enabling
hardware-backed HTTP signature authentication for hosts.
* Introduced HTTP signature verification middleware for API requests,
applied conditionally for premium licenses.
* Hosts presenting identity certificates must authenticate with matching
HTTP message signatures during enrollment and authentication.
* Added SCEP-based certificate issuance for secure host identity
management.
* Updated enrollment endpoints to use standardized request/response
contract types.
* **Bug Fixes**
* Enhanced authentication logic to verify consistency between host
identity certificates and host records, preventing duplicate or
mismatched identities.
* **Chores**
* Updated dependencies and test infrastructure to support HTTP signature
verification and host identity certificate workflows.
* Added comprehensive integration and datastore tests for host identity
certificate issuance, storage, and authentication.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
