Support `unknown` as value for the `is` postgrest filter

[Vinzent03]

Description

The current type of the value for the isFilter is bool? which does not allow specifying unknown even though that is supported by postgrest: https://docs.postgrest.org/en/v12/references/api/tables_views.html#operators

supabase-flutter/packages/postgrest/lib/src/postgrest_filter_builder.dart

Line 227 in 6e11db6

PostgrestFilterBuilder<T> isFilter(String column, bool? value) {

Suggested solution

I propose to add a new enum with four values for this. While bool? is a great type for the other values: null, true, false it does now work for the remaining one unknown. This also prevents from accidentally using null.
This would be a breaking change for v3.

Alternative

No response

Additional context

No response

Validations

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[spydon]

Oufff, I'm not too keen on replacing the bool with an enum...
What about if we added a special operation just for unknown? I would think that it is a lot less commonly used than the other values.
So it would just look something like this:

/// A check for the SQL boolean `UNKNOWN` state.
///
/// Only meaningful on boolean columns/expressions. See [isFilter] for
/// `null`, `true`, and `false`.
PostgrestFilterBuilder<T> isUnknown(String column) {
  return copyWithUrl(appendSearchParams(column, 'is.unknown'));
}

[Vinzent03]

Yeah I get that and sounds reasonable. Will do that after #1610 is merged.