ENH: return label and description fields from read_raw_egi

[obergmartin]

Reference issue

Fixes #10121

A resurrection!

What does this implement/fix?

This PR ensures that read_raw_egi() returns the label and description fields from EGI's Events_ECI TCP-IP 55513.xml file as a dictionary in the Annotations.extra field.

I wrote this code myself, no AI.

Additional information

I feel it's good to populate the Annotations.extra field directly here in the events.py file as opposed to reading the file a second time from an external function and ensuring that timestamps match up.

I changed the data type returned in event_tims by _read_mff_events() from a list to a dict. I felt this was better than returning an extra variable.

Since the original #10121, I see #8038 has been opened and this data is being read by mffpy. These changes allow the eventTrack data to make its way into Annotations.

[larsoner]

Would it be possible to modify some existing test to show this works? Even something like

assert raw.annotations[0]["extras"]["label"] == ...

or whatever you need to do to show that some info is being pulled in would be great

Also there are some failures in CIs like

_______________________ test_io_egi_pns_mff_bug[False] ________________________
mne\io\egi\tests\test_egi.py:354: in test_io_egi_pns_mff_bug
    raw = read_raw_egi(
<decorator-gen-480>:10: in read_raw_egi
    ???
mne\io\egi\egi.py:176: in read_raw_egi
    return _read_raw_egi_mff(
<decorator-gen-477>:10: in _read_raw_egi_mff
    ???
mne\io\egi\egimff.py:362: in _read_raw_egi_mff
    return RawMff(
<decorator-gen-478>:10: in __init__
    ???
mne\io\egi\egimff.py:584: in __init__
    self.set_annotations(Annotations(**annot))
                         ^^^^^^^^^^^^^^^^^^^^
mne\annotations.py:412: in __init__
    _check_o_d_s_c_e(onset, duration, description, ch_names, extras)
mne\annotations.py:204: in _check_o_d_s_c_e
    extras = _validate_extras(extras, len(onset))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
mne\annotations.py:153: in _validate_extras
    raise ValueError(
E   ValueError: extras must be None or a list of length 1, got 0.