Ab/implement multi base custom reader

[abarciauskas-bgse]

This PR:

• removes the items endpoints since we don't need them yet
• adds the AssetReader class which inherits from MultiBaseReader and enables dynamic definition and use of the asset reader depending on the asset type. Mostly a copy paste of the implementation of the tile method for the MultiBaseReader in enable dynamic definition of Reader for MultiBaseReader cogeotiff/rio-tiler#711. The XarrayReader is not implemented yet so the _get_reader method will throw an error for the hdf5/netcdf media types
• Adds a few tests

A few questions / todos which have been turned into new issues:

• Fix test for tile method in tests/test_assets_reader.py (very slow) #5
• Declare required keys bbox and assets of input to AssetReader #6
• Tests for index request query parameters #7

[abarciauskas-bgse]

this is very slow for some reason, even though as far as I can tell the mock_rasterio_open is working as expected.

[abarciauskas-bgse]

I removed this context manager because I saw one was being used in AssetReader#tile#_reader with self.ctx but should we have it here as well since as you mentioned there are 3 levels of reader (STAC, item and asset)?

[abarciauskas-bgse]

I reverted this change as I could see why different context manager blocks are needed since the env passed in the request could be different than the env passed in the asset_info, but I would be curious to know it's intentional to have multiple nested rasterio.Env context managers.

[vincentsarago]

it's intentional to have an env for each reader, so we can set variable like GDAL_INGESTED_BYTES_AT_OPEN using specific number

[abarciauskas-bgse]

@vincentsarago I believe I have addressed all comments, do you mind taking another look?

There are some outstanding issues mostly around testing and type checking that I listed in the description that I will turn into issues.