Allow disabling addition of brackets in the signature mangling in autosummary

The autosummary extension mangles signatures in the summary table by adding brackets to signatures whenever there are optional arguments. For example, a signature like

def func(x, *, arg1, arg2):

will be mangled to

func(x, *[,arg1, arg2])

This behavior is hard-coded in the autosummary code here. The only way to disable it is to monkeypatch mangle_signature in conf.py, like

import sphinx.ext.autosummary as autosummary_mod
if hasattr(autosummary_mod, '_module'):
    # It's a sphinx deprecated module wrapper object
    autosummary_mod = autosummary_mod._module
autosummary_mod.mangle_signature = lambda sig, max_chars=30: sig

I would prefer for the table to not add the brackets and ellipses to the signatures, as these are not valid Python code, and don't add anything to the original valid-Python signature. Frankly I think this behavior should not be there at all, but there should at least be an option to disable it.

Start in sphinx/ext/autosummary/init.py at the mangle_signature call around the linked line, then trace how autosummary options are read. Add a configuration option that controls bracket and ellipsis insertion, and verify that disabling it preserves the original valid-Python signature in summary tables.