define titiler multiscales consumer API

[d-v-b]

to shield consumers like titiler from flux in the multiscales metadata, we could define an API layer that maps zarr metadata to a consistent data structure that titiler can use.

When a new version of multiscales metadata is released, we would also release a new version of this API, to ensure that new data is compatible with titiler (assuming titler updates its dependency on this API).

[d-v-b]

cc @vincentsarago