More granular control of starting offsets (assign)

Right now you can only run a Streaming Query starting from either the earliest or latests offsets available at the moment the query is started. Sometimes this is a lot of data. It would be nice to be able to do the following:

• seek to user specified offsets for manually specified topicpartitions

currently agreed on plan:

Mutually exclusive subscription options (only assign is new to this ticket)

.option("subscribe","topicFoo,topicBar")
.option("subscribePattern","topic.*")
.option("assign","""{"topicfoo": [0, 1],"topicbar": [0, 1]}""")

where assign can only be specified that way, no inline offsets

Single starting position option with three mutually exclusive types of value

.option("startingOffsets", "earliest" | "latest" | """{"topicFoo": {"0": 1234, "1": -2}, "topicBar":{"0": -1}}""")

startingOffsets with json fails if any topicpartition in the assignments doesn't have an offset.