SPARKNLP-924: DocumentCharacterTextSplitter

[DevinTDHa]

Description

This PR introduced the DocumentCharacterTextSplitter which allows users to split large documents into smaller chunks.

DocumentCharacterTextSplitter takes a list of separators. It takes the separators in order and
splits subtexts if they are over the chunk length, considering optional overlap of the chunks.

Motivation and Context

This will allow users manage large texts better to feed into e.g. language models

How Has This Been Tested?

New and old tests are passing

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code improvements with no or little impact
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING page.
• I have added tests to cover my changes.
• All new and existing tests passed.