Support for multiline (custom height) validation (and bottom toolbar) messages

I would like to be able to add multiline error messages at the bottom of prompts to enable more detailed feedback (e.g. pydantic validation for users inputting more complex data structures (e.g. yaml, json)

Context: I've followed the path upstream with the aim off adding multiline error messages to https://github.com/copier-org/copier, which uses https://github.com/tmbo/questionary, which in turn uses this prompt toolkit.

Current workaround for this is an extension that monkey patches prompt toolkit: https://gitlab.com/cbrown1234/copier-pydantic, which is a fairly dirty hack.

import prompt_toolkit

_validation_toolbar_init = prompt_toolkit.widgets.ValidationToolbar.__init__


def _validation_toolbar_init_patched(
    self: prompt_toolkit.widgets.ValidationToolbar,
    *args: tuple,
    **kwargs: dict,
) -> None:
    """Wrap ValidationToolbar __init__ for patching."""
    _validation_toolbar_init(self, *args, **kwargs)

    # Uncap validation message length to enable multiline error messages
    self.container.content.height = None

I'm hoping to contribute proper support by updating the interface of PromptSession (because questionary already passes through extra **kwargs to PromptSession to enable customisation, e.g. https://github.com/tmbo/questionary/blob/f28b2d1f0c0d4a37e2a08c1627834366be6d1c17/questionary/prompts/text.py#L97)

I've explored a little how this could work, and will raise a draft MR, but given the number of other toolbars you might be looking for a more scalable design

Start by reading the ValidationToolbar and PromptSession interfaces in prompt-toolkit, then compare how questionary's prompts/text.py passes customization through. Trace the other toolbar implementations to determine a scalable configuration approach, and consider the work complete when multiline validation and bottom-toolbar messages can be configured without monkey patching.