Support Cluster Peering in the service / services API functions

[jcjones]

Consul 1.14 added support for Cluster Peering, and the new peer parameter for the /health API endpoint to access imported services.

It would be natural for consul-template to support the new peer parameter for the service and services API functions. This would allow consul-template to offer imported services in configuration files

The current service function syntax is:

{{ service "<TAG>.<NAME>@<DATACENTER>~<NEAR>|<FILTER>" }}

Perhaps simply double-@s like @@<PEER>, or a character like !<PEER> would suffice and stay short, or add convention for peer=<PEER>?

The code changes for this seem to be mostly in https://github.com/hashicorp/consul-template/blob/main/dependency/health_service.go.

[blake]

Support for querying peered services was added by #1822.