Extend metadata filtering support in ElasticsearchDocumentStore

[bogdankostic]

This PR allows users to apply a wide variety of filters on Documents inside their ElasticsearchDocumentStore.

Filters are defined as nested dictionaries. The keys of the dictionaries can be a logical operator ("$and", "$or", "$not"), a comparison operator ("$eq", "$in", "$gt", "$gte", "$lt", "$lte") or a metadata field name.

Logical operator keys take a dictionary of metadata field names and/or logical operators as value. Metadata field names take a dictionary of comparison operators as value. Comparison operator keys take a single value or (in case of "$in") a list of values as value.

If no logical operator is provided, "$and" is used as default operation.
If no comparison operator is provided, "$eq" (or "$in" if the comparison value is a list) is used as default operation. Therefore, we don't add breaking changes.

The tests pass when using an OpenSearchDocumentStore as well, so this filtering functionality can als be used right away with OpenSearch.

Example:

filters = {
    "$and": {
        "type": {"$eq": "article"},
        "date": {"$gte": "2015-01-01", "$lt": "2021-01-01"},
        "rating": {"$gte": 3},
        "$or": {
            "genre": {"$in": ["economy", "politics"]},
            "publisher": {"$eq": "nytimes"}
        }
    }
}
# or simpler using default operators
filters = {
    "type": "article",
    "date": {"$gte": "2015-01-01", "$lt": "2021-01-01"},
    "rating": {"$gte": 3},
    "$or": {
        "genre": ["economy", "politics"],
        "publisher": "nytimes"
    }
}

Open To-Do DONE

With the current implementation, we can't use the same logical operator twice on the same level, i.e., the following filter is not valid as dictionary keys have to be unique:

{"$or":
    {"$and": {
         "Type": "News Paper",
         "Date": {"$lt": "2019-01-01"},
     "$and": {
         "Type": "Blog post",
         "Date": {"$gte": "2019-01-01"}
    }
}

To be able to handle the above mentioned case, we will allow logical operators to optionally take a list of dictionaries as value, such that the above filter would like this:

{"$or": [
    {"$and": {
        "Type": "News Paper",
        "Date": {"$lt": "2019-01-01}"
    }},
    "$and": {
        "Type": "Blog post",
        "Date": {"$gte": "2019-01-01"}
    }}   

]}

(Thanks to @MichelBartels for spotting this!)

Closes #2029

[tholor]

Looking good. Left a few minor comments.

[tholor]

probably I missed this one, but where does this method get implemented? I saw it being called in the ElasticsearchDocumentStore...

[bogdankostic]

This method gets implemented in each of the LogicalOperation and FilterOperation classes, such that each operation is responsible for converting their part of the whole filter clause.

[tholor]

Please also add some labels to the PR before merging