Accept Callables as Tokenizers for InMemoryDocumentStore

[farhanhubble]

InMemoryDocumentStore currently only accepts a tokenizing pattern through the argument bm25_tokenization_regex: str = r"(?u)\b\w\w+\b". The underlying BM25 supports a callable though. Removing this restriction will enable correct tokenization of a larger variety of corpora. I ran into this limitation trying to index JSON documents that contain key-value pairs, like:

"casNumber": "96-80-0", 
  "concentration": "<= 100 %", 
  "detailedConcentration": {
    "approximate": false, 
    "fromConcentration": 0, 
    "fromOperand": "", 
    "remainder": false, 
    "toConcentration": 100.0, 
    "toOperand": "<=", 
    "unavailable": false
  }, 
  "ecNumber": "202-536-2", 
  "molecularFormula": "C8H19NO", 
  "molecularWeight": "145.24 g/mol"

[masci]

Thanks for the feedback, I'm going to create a feature request from this discussion, contributions are welcome!