WIP: Add decode_timedelta argument to open_virtual_dataset

[jbusecke]

I have started an implementation here by simply following through the code and adding decode_timedelta wherever decode_times was passed along.

I noticed that the type hints only allowed for bool|None previously, but xarray allows passing CFTime/TimedeltaCoder and per variable mappings. I have added these here to all relevant functions for now, but can revert this if I am missing the point of it only being bool|None.

It seems that I will have to dig a bit deeper into the typing to make this pass, but before I venture that route it would be great to have a chat about this.

What I am missing here are additional tests. I might need a bit of help/input to make sense of what exactly we should test here.

• Closes Unable to provide decode_timedelta to open_virtual_dataset #749
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

[maxrjones]

@jbusecke I think that the more limited typing in VirtualiZarr is just due to the function pre-dating the new typing in Xarray (added in pydata/xarray#9901). I recommend using the newer typing.