Add ReadAll transform for MongoDBIO

To enable composable reads, for example to start reading data in the middle of a Pipeline we need to offer a ReadAll transform for MongoDBIO, ideally this should include a refactor of the current Read. For details of how this can be done take a look at the changes in JdbcIO, RedisIO and SolrIO.

Mailing list discussion: https://lists.apache.org/thread.html/7bd4b9533ecfb5236969567f6436f46f4743eda04eaf32ef15fc7ee3@%3Cdev.beam.apache.org%3E