This PR introduces the CE plumbing for a new high WritePriority, meant
to bypass rejection from the AOP write controller. We attach this
priority to any request on a sudo path, such that administrators can
still perform necessary operations during an overload.
This PR introduces the CE plumbing for a new HTTP header, called
X-Vault-AOP-Force-Reject, which will force any associated request to
reject storage writes as if Vault were overloaded.
This flag is intended to test end-to-end functionality of write
rejection in Vault. This is specifically useful for testing 503 -
Service Unavailable HTTP response codes during load shedding.
