Initial commit of boilerplate of change stream pipeline for bigtable

[tonytanger]

Adding the skeleton of the change stream pipeline of Google Cloud Bigtable. The logic is similar to that of Cloud Spanner's change stream connector.

Pipeline contains 3 steps

• InitializeDoFn - Responsible to validating preconditions such as creating metadata table, ensuring options are valid and correct, and determine a valid start time for the pipeline.
• DetectNewPartitionsDoFn - Manages the lifecycles of partitions being streamed. This includes initially outputting the initial list of partitions to be streamed, and as partitions split and merge, create new partitions to be streamed. DetectNewPartitions is also responsible to ensure all partitions are streamed and non are missing and keep track of the watermark of the entire pipeline.
• ReadChangeStreamPartitionDoFn - Streams individual partitions and output data changes. It also handles termination, whether that's the end of the pipeline termination, or the partition has split or merged so it no longer exists.

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[pabloem]

thanks for the good docs here. Do you think it makes sense to add some more documentation strings to the header of this file? e.g. links to how to start a changestream, the sort of data the transform outputs, the expected throughput, how to use with Beam schema (if supported), etc?

happy to leave for next PR, but I think it will be important.