Remove quotes around placeholders in Elasticsearch custom query

[tanaysoni]

This PR removes quotes around placeholders in Elasticsearch custom_query parameter.

Before

The placeholder terms($query, $years) had quotes around them. For instance,

retriever = ElasticsearchRetriever(
        document_store=document_store,
        custom_query="""
                {
                    "size": 10, 
                    "query": {
                        "bool": {
                            "should": [{
                                "multi_match": {"query": "${query}", "type": "most_fields", "fields": ["text"]}}],
                                "filter": [{"term": {"year": "${years}"}}]}}}"""
)

Now

The placeholders must be used without quotes. For instance,

retriever = ElasticsearchRetriever(
        document_store=document_store,
        custom_query="""
                {
                    "size": 10, 
                    "query": {
                        "bool": {
                            "should": [{
                                "multi_match": {"query": ${query}, "type": "most_fields", "fields": ["text"]}}],
                                "filter": [{"term": {"year": ${years}}}]}}}"""
)

[tholor]

Why is it "${query}"here but one line afterwards it's only ${years} without quotation marks?

[tanaysoni]

Yes, that's inconsistent. I removed quotes around the query placeholder as well.

[tholor]

LGTM

[tholor]

Can you please add a "before vs. after" code snippet for the breaking change (so that we can easily copy & paste it to the next release notes)?