Expose the blocking `RawTextReader`'s `new_with_size` through the Builder API.

[nirosys]

text::RawTextReader offers a function to create a new reader with a user provided size hint. The hint acts as the initial buffer size, as well as the default read size. Both can grow if needed, but the initial hint allows the user to tune the reader slightly to their expectations.

This functionality should be exposed through the builder API.