Fix typo in similarity docs

DRF similarity -> DFR similarity
Allow specifying nested fields in simple_query_string

Fixes #5091
[docs] fixed errors in aggs docs
- error in nested aggs example
- error in terms aggs example
Add mustache templating to query execution.

Adds support for storing mustache based query templates that can later be filled
with query parameter values at execution time.

Relates to #4879
Rename template query parameter names.

This makes the names of the parameters a template query accepts shorter and
clearer.

Relates to #4879
Refactoring to allow for non quoted templates.

As per suggestion in #4879 modified the implementation to allow for templates
that are not quoted but legitimate json. Leaving the option to completely quote
in - needed anyway for support for referencing templates stored in the config
directory.

Relates to #4879
Mark mustache as optional dependency.

Relates to #4879
Fixing pull request comments by s1monw.

Relates to #4879