support filter query parameter

[newgene]

Difference between query and filter context in an ES query:

https://www.elastic.co/guide/en/elasticsearch/reference/current/query-filter-context.html

The query placed in the filter context can be faster than in the normal query context (e.g. passed to q query parameter):

• filter query does not included in the _score calculation
• filter query can be cached by ES

When implemented, we can change this API query:

?q=object.umls:C0872079 AND pmid_count:>5 AND predicate:LOCATION_OF

to

?q=object.umls:C0872079&filter=pmid_count:>5 AND predicate:LOCATION_OF

They should return the same list of hits, but it would be preferred if the part of filter query is used repetitively while the q query changes.

[newgene]

filter query parameter should work for both GET and POST for the query handler.

Note that we currently used filter to be an alias of fields parameter, for the back-compatiblity, but it's no longer an issue, so we can remove that alias and reuse it for different functionality.

[newgene]

We already have a similar implementation in one of API instance:

https://github.com/NIAID-Data-Ecosystem/nde-discovery-api/blob/08dfae752613b45647f8ce83350466733ea1e6e6/nde-web/pipeline.py#L64 (called extra_filter but does the same feature)

[newgene]

Also want to note that this is related to post_filter query parameter from #208. The key difference is post_filter does not impact the aggregation results, while filter does, so typically, these two related query parameters will be used in difference scenarios.