Nomad does not support agent service health checks when tls.verify_https_client is enabled

### Nomad version
v0.10.2

### Issue

The Nomad TLS doc at https://www.nomadproject.io/guides/security/encryption.html#network-isolation-with-tls says "Consul will not attempt to health check agents with verify_https_client set as it is unable to use client certificates." However, the Consul docs for health checks indicate that health checks do support TLS certificates. See https://www.consul.io/docs/agent/checks.html and https://www.consul.io/docs/agent/options.html#enable_agent_tls_for_checks).

We should take advantage of Consul's ability to use its certificates for doing health checks. Its possible, perhaps likely, that Consul's certificate is invalid for accessing Nomad, so we need an alternative method (eg Nomad could create a TTL check and heartbeat).

Start with the Nomad TLS documentation and the linked Consul agent check and options documentation, focusing on verify_https_client and enable_agent_tls_for_checks. Trace the Nomad and Consul integration entry points for agent health checks. Done means Nomad supports agent health checks when client verification is enabled, including a defined approach for certificate validation or an alternative health-check mechanism.

Written by the indexing model from the issue text.