Allow .attrs to use dict-likes

Is your feature request related to a problem? Please describe.
Reading attributes from h5py-files is rather slow. So instead of retrieving it immediately I wanted to create a lazy dict-class that only retrieves the attribute values when necessary. But this is difficult to achieve since xarray keeps forcing the attrs to dicts in a lot of places.

Describe the solution you'd like

• Replace in https://github.com/pydata/xarray/blob/dddac11b01330791ffab4dfc72d226e71821973e/xarray/core/variable.py#L865 and https://github.com/pydata/xarray/blob/dddac11b01330791ffab4dfc72d226e71821973e/xarray/core/dataset.py#L798 with a asdict(value) function that checks if the input is a valid dict-like, if not convert to dict. Things that might be good to check:
  • MutableMapping
  • hasattr(dict_like, "copy")
  • isinstance(dict_like, dict) == True
• Remove unneccessary conversions to dict. For example https://github.com/pydata/xarray/blob/dddac11b01330791ffab4dfc72d226e71821973e/xarray/core/merge.py#L523 should not be necessary as attrs from variables/dataarrays/datasets have already been forced to dicts when they were initialized.

Describe alternatives you've considered

• One could lazify with dicts as well, for example by replacing the value with a function. This however won't look good in reprs, that's why having a convienence class is nice.
• dict(LazyDict) always forces to dict, it does not let it pass through unchanged even if isinstance(LazyDict, dict) == True.

Interesting reading:
https://stackoverflow.com/questions/16669367/setup-dictionary-lazily
https://stackoverflow.com/questions/3387691/how-to-perfectly-override-a-dict

Start with xarray/core/variable.py around line 865, xarray/core/dataset.py around line 798, and xarray/core/merge.py around line 523. Review how attrs are converted and determine the dict-like behavior described in the issue; done means supported lazy dict-like attrs are preserved where appropriate and unnecessary conversions are removed.