Handle CHAR data type in the backend.

[kgodey]

This issue is to ensure that Mathesar can handle the Postgres CHAR data type.

As part of this issue, we need to ensure that:

• Users can use the API to change a column to CHAR if it's possible to do so.
• Users can set and change the n (length) of a given CHAR column via a type_options field in the API.
  Additional Context
• See PR Extend the v0/databases/'s types list to show both Mathesar and DB type information. #368 for code that calculates which types are allowed for a column.
• No changes are needed for type inference; I don't think we should be suggesting a restrictive type like CHAR.