Allow for preferential bridge assignment to buckets
After #9652 (moved) is fixed, we should also provide a way to specify which transports a bucket prefers.
Something along the line of:
FILE_BUCKETS = { "name1": {"size": 10, "pt": ["obfs2", "obfs3"]},
"name2": {"size": 15},
"foobar": {"size": 3, "pt": ["obfs3"]}}
Which will result in the "name1" bucket containing 10 bridges, where most will support obfs2 and/or obfs3. "name2" bucket will contain 15 bridges, but has no preference for pluggable transports, and "foobar" will contain 3 bridges where most will be obfs3 bridges.