Elasticsearch Output: Support using tag in template name

## Feature Request

Opening a feature request kicks off a discussion.

### Proposal: Support using tag with notation `{{tag_name}}` in template name

### Current behavior: `index_name` supports using tag as part of the name. But `template_name` can only be hardcoded.

```ini
[global_tags]
monitor = "http"
[[outputs.elasticsearch]]
urls = [ "http://127.0.0.1:9200" ]
index_name = "telegraf-{{monitor}}-%Y.%m.%d"
template_name = "telegraf-http"
```

### Desired behavior: Both `index_name` and `template_name` support tag notation

```ini
[global_tags]
monitor = "http"
[[outputs.elasticsearch]]
urls = [ "http://127.0.0.1:9200" ]
index_name = "telegraf-{{monitor}}-%Y.%m.%d"
template_name = "telegraf-{{monitor}}"
```

### Use case:

When the `global_tags.monitor` is changed, the hardcoded `template_name = "telegraf-http"` may be forgotten.

If `template_name` supports tag reference, we only need to change the `global_tags.monitor` and the `template_name` will be modified automatically.

Start by locating the Elasticsearch output implementation and the existing handling of index_name and template_name. Trace how tag notation is expanded, then make template_name follow the same behavior and verify that the configuration examples produce matching names when the monitor tag changes.

Written by the indexing model from the issue text.