[TSVB] Runtime fields not fully working with annotations

[immavalls]

Kibana version:
7.13.2

Elasticsearch version:
7.13.2

Server OS version:
N/A

Browser version:
N/A

Browser OS version:
N/A

Original install method (e.g. download page, yum, from source, etc.):
ESS / Elastic Cloud

Describe the bug:

When creating a TSVB, if the index pattern used for Annotations has a runtime field, it works for the query string, but it's not recognized in the markdown.

Steps to reproduce:

1. Create a TSVB with any index that has a runtime field. e.g. day_of_week as in https://www.elastic.co/guide/en/elasticsearch/reference/master/runtime-mapping-fields.html#runtime-mapping-fields
2. We can use it in the query string day_of_week: "Thursday"
   
3. But if we use it in the fields / row template, it is not recognized Error processing your markdown {{day_of_week}} is un unknown variable
   

Expected behavior:
Same as a regular field, to be able to use it in the annotations markdown.

Any additional context:
Runtime fields supported since 7.13.0 as per #82438

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)