[Feature Request]: provide option for explicit schema data types for batch input sources and callable functions

[ewanzhang-google]

What would you like to happen?

In a few scenarios beam has failed to interpret the data type, these include:

1. a source coming from BigQuery table, the unique id column with alphanumeric value (type string) or a timestamp column with millisecond integer value (type string) can't be decoded
2. a python callable function that create a new column, the column type can't be decoded even when it's explicitly casted in the function body

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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