Allow `fsaverage` for `subject` in `mne.Report.add_trans`

[sappelhoff]

https://mne.tools/stable/generated/mne.Report.html#mne.Report.add_trans

Currently, only a path-like or a trans object are permissible:

mne-python/mne/report/report.py

Lines 1607 to 1610 in f6c6bd7

	Parameters
	----------
	trans : path-like | instance of Transform
	The ``head -> MRI`` transformation to render.

☝ however, throughout the mne code base, "fsaverage" is a commonly accepted trans argument.

Would it be very hard to allow for "fsaverage"?

[hoechenberger]

I think supporting it makes sense and I believe adding support is going to be trivial, want to give it a shot?