Provide a withReadableByteChannelFactory on TextIO.Read

Hi,

I am prototyping a dataflow job that reads from GPG encrypted files. I got this working, but had to jump through some hoops to supply a new "compression" format to TextIO: I basically had to copy a good part of TextIO.Read only to add a `withDecompressingChannelFactory` method similar to `withWritableByteChannelFactory` in TextIO.Write.

Would it be possible to add a method `withDecompressingChannelFactory` or `withReadableByteChannelFactory` to TextIO.Read to allow users to plug in custom decompression/decryption code?

Imported from Jira [BEAM-3262](https://issues.apache.org/jira/browse/BEAM-3262). Original Jira may contain additional context.
Reported by: Tobi19.

Start by reading the TextIO.Read API and compare it with the withWritableByteChannelFactory method on TextIO.Write. Determine how a readable channel factory should support custom decompression or decryption, and consider the issue complete when TextIO.Read exposes the requested factory hook for those use cases.

Written by the indexing model from the issue text.