Added max fields and doc size limits

[pandu-k]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behaviour? (You can also link to an open issue here)
No way to limit the size of documents and the max number of fields per index.

What is the new behaviour (if this is a feature change)?
A size of document limit and the max number of fields per index limit are now settable by env var.

The doc size limit has a default size of 100k bytes.

Localhost Marqo-OS requests no longer raise warnings.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Non-breaking data model changes:

• Text fields no longer produce the automatically created keyword field, in the parent doc
• Very large fields get their keyword (for filtering the facet object) ignored, rather than Marqo-OS preventing the doc being indexed

Other information:
Ran tox