Mutable FST Support

This issue tracks the implementation of mutable FST, which will allow Pinot to perform in-memory, real time text searches using text indices, while allowing the index to grow and mutate. The new data will be available to search without any lag, and not requiring a flush.

Please refer to the design document:

https://docs.google.com/document/d/1O2ttsplFAflkM1Q-8-7yRNrD9EgCCWYm-W63NdC7ghE/edit?usp=sharing

No files, tests, or entry points are named. Start by reading the linked design document and use it to identify the affected Pinot text-index components. Done means an FST can grow and mutate in memory, with new data searchable immediately without a flush.

Written by the indexing model from the issue text.