Support multiple resource buckets

[nawatts]

This adds support for multiple resource buckets in order to provide access to resources hosted on different cloud providers through our resources framework (#255, #263, #264).

This adds a GnomadPublicResource resource class and subclasses for each resource type (GnomadPublicTableResource, GnomadPublicMatrixTableResource, etc). When reading data, GnomadPublicResource rewrite their path attribute based on the value of gnomad.resources.config.gnomad_public_resource_configuration.source.

Thus, to read resources from gcp-public-data--gnomad instead of gnomad-public-requester-pays,

from gnomad.resources.config import gnomad_public_resource_configuration, GnomadPublicResourceSource
from gnomad.resources.grch37 import gnomad

gnomad_public_resource_configuration.source = GnomadPublicResourceSource.GOOGLE_CLOUD_PUBLIC_DATASETS

ds = gnomad.public_release("exomes").ht()

Alternatively, a custom path can be used (for example, if someone has created their own copy of the gnomAD public buckets).

from gnomad.resources.config import gnomad_public_resource_configuration, GnomadPublicResourceSource
from gnomad.resources.grch37 import gnomad

gnomad_public_resource_configuration.source = "gs://my-bucket/gnomad-resources"

ds = gnomad.public_release("exomes").ht()

[nawatts]

Couldn't figure out why Pylint insisted this was an error. The tests show that this works.

Pylint also only complained about this when running in the PR checks. It passed when running Pylint locally.

[jkgoodrich]

Mostly questions for my knowledge.

[jkgoodrich]

just one comment, but lgtm. Thank you for answering all my questions and adding all the extra tests!

[jkgoodrich]

add return type

[nawatts]

Had to rebase to resolve merge conflicts between the from typing import ... added in bc1b495 and the module docstring added in #372.