Disable removing dynamic default value for primary keys, allow editing PK column in new un-saved rows if no default value is set

[pavish]

Current behavior

• We allow editing/removing default values for all columns, including PK columns.
• We do not allow editing cells for PK columns.

Bugs due to the current behavior

• Users are able to remove a dynamic default value for PK columns. Once removed, adding any new row will fail.
• If no default value is present for a PK column, adding a new row will fail. Since we do not allow editing PK column cells, users will never be able to add a new row.

Expected behavior

• Users should not be able to remove/modify a dynamic default value of a PK column.
• Users should be able to edit a PK column cell in an un-saved row. They should not be allowed to do that for saved rows.

[seancolsen]

Related: #1362, #1361

[pavish]

Blocked by https://github.com/centerofci/mathesar/issues/1672