feat: tantivy_0.20.1_upgrade

[Sidhant29]

This Pr reflects the API changes observed from tantivy 0.19.2 to 0.20.1.

• The numeric_options interface has changes from requiring a str param of 'single', 'multivalued', to just a bool param. Fast fields hence support only 'single' mode.
• The order_by_u64 method now accepts a 'str' field name, instead of the entire field object.
• The schema.get_field method returns a Result instead of an option.

[cjrh]

I had a look too, same question as @adamreichold why did the test change?

Resolved. We figured out it was the writer heap setting. More info in the other resolved comment on this PR.