Don't assume nilable when #__tapioca_type is provided

[rzane]

Motivation

#__tapioca_type is meant to serve as an escape hatch to define arbitrary types for ActiveModel attributes. Currently, this escape hatch is limited, because it still assumes that all attributes are nilable.

The #__tapioca_type method should have full control over what type is emitted to the RBI file.

Implementation

This PR adds .assume_nilable? to ActiveModelTypeHelper. This method will only return false if the ActiveModel type implements #__tapioca_type.

It is the responsibility of the person who defined #__tapioca_type to wrap the type in T.nilable.

Alternatives:

1. Allow subclasses of ActiveModel::Type::Value to implement #__tapioca_nilable?.
2. Call .as_nilable_type in ActiveModelTypeHelper.type_for to avoid adding another public method. This is probably the simpler solution, but it would updates to the expected return values in tests.

Tests

This PR includes tests.