Allow index field type feature to configure dynamic_templates

## What?

Allow index field type feature to configure dynamic_templates:

For example (excerpt from `graylog-internal`:

```json
{
"graylog-internal": {
"order": 0,
"index_patterns": [
"graylog_*"
],
"mappings": {
"dynamic_templates": [
{
"internal_fields": {
"mapping": {
"type": "keyword"
},
"match_mapping_type": "string",
"match": "gl2_*"
}
}
]
},
"aliases": {}
}
}
```

## Why?

This allows setting multiple fields using a match pattern. This is useful if the Graylog environment has a large number of fields or may not be able to predict what the field names will be ahead of time. Some reasons this may happen if the log source is part of an application developed by the customer and logged fields can change, or flattening json.