Ability to Use Prepared Query in Place of Service

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### Consul Template version

```
consul-template v0.18.5 (9902dd5)
```

### Configuration

```hcl
N/A
```

```liquid
upstream exampleservice {
least_conn;
keepalive 16;

#Attempt to fetch the exampleservice servers
{{if service "exampleservice"}}
{{range service "exampleservice"}}server {{.Address}}:8443;
{{end}}
{{else}} # upstream stanza must have a server directive
server 127.0.0.1 down;
{{end}}
}
```

### Command

```shell
consul-template --template=/etc/templates/etc/nginx/virtual-servers/pws.conf:/etc/nginx/virtual-servers/pws.conf:systemctl reload nginx --template=/etc/templates/etc/nginx/locations/pws.conf:/etc/nginx/locations/pws.conf:systemctl reload nginx --template=/etc/templates/etc/nginx/upstreams/pws.conf:/etc/nginx/upstreams/pws.conf:systemctl reload nginx --wait=5s:10s
```

### Debug output

N/A

### Expected behavior

This is a feature request, so what I'd like to be able to do is something like this:

```liquid
upstream exampleservice {
least_conn;
keepalive 16;

#Attempt to fetch the exampleservice servers
{{if query "exampleservice"}}
{{range query "exampleservice"}}server {{.Address}}:8443;
{{end}}
{{else}} # upstream stanza must have a server directive
server 127.0.0.1 down;
{{end}}
}
```

### Actual behavior

In the absence of using prepared queries, we are constrained to only having services within a given datacenter with the configuration as it exists. We could define a second section, with a hard coded backup:

```liquid
upstream exampleservice {
least_conn;
keepalive 16;

#Attempt to fetch the exampleservice servers
{{if service "exampleservice"}}
{{range service "exampleservice"}}server {{.Address}}:8443;
{{end}}
{{else}} # upstream stanza must have a server directive
server 127.0.0.1 down;
{{end}}

#Attempt to fetch the backup exampleservice servers
{{if service "exampleservice@guse4-poc"}}
{{range service "exampleservice@guse4-poc"}}server {{.Address}}:8443 backup;
{{end}}
{{else}} # upstream stanza must have a server directive
server 127.0.0.1 down;
{{end}}
}
```

But that gets tricky to maintain, and query seems like the perfect solution for this, if it were an option to iterate over, as service is.

### Steps to reproduce

N/A

### References

Are there any other GitHub issues (open or closed) that should
be linked here? For example:
N/A

No files, tests, or entry points are named. Start by examining the existing `service` template behavior and the requested Consul prepared-query behavior; done means templates can conditionally iterate over query results as shown, while preserving the fallback configuration pattern.

Written by the indexing model from the issue text.