Allow to format signatures in docstrings

[krassowski]

This is a proposal for #630. It comes down to:

• adding pylsp_signatures_to_markdown hook which takes a list of strings (signatures) and returns a single (markdown) string
• reading the hook value and passing it down as signatures_to_markdown to format_docstring

Using a list → str approach allows the hook to modify how alternative signatures are displayed.

There are no user-facing changes.

[krassowski]

Any thoughts @ccordoba12 ? It would be amazing to get it in to allow downstreams to customize.