Set _stac_io on constructed object if root passed into from_dict

[matthewhanson]

The from_dict functions can now take in a root catalog. If provided, the resulting object should set it's _stac_io object to that of the root.

As it stands now, if you get Collections from a root catalog, those collections will switch back to using the DefaultStacIO class.

[matthewhanson]

@duckontheweb This was changed in #590, however now I see that this initial ticket I created is flawed. A couple weeks @lossyrob and I talked briefly about this and now that I've dug into this again I see he's correct.

The issue was due to pystac-client incorrectly accessing self._stac_io when it should be accessing the root Catalog's _stac_io. For some reason I implemented the CollectionClient.assert_conforms_to referencing the instances _stac_io attribute...but no other class in PySTAC does this. Resolving STAC objects happens through the entities root _stac_io attribute, and CollectionClient.assert_conforms_to should do the same.

I'm implementing this now in pystac-client, and will remove the from_dict workaround I had to set the CollectionClient _stac_io attribute and test with PySTAC main, and will report back.

I apologize for requesting this change, but I think it should be reverted.