Allow using side inputs as ValueProviders

Allow dynamically computed information in a pipeline which is materialized as a side input to be used in the same places as ValueProviders.

This could solve the problem mentioned in [https://stackoverflow.com/questions/45649244/google-cloud-dataflow-apache-beam-can-i-use-sideinputs-with-textio-write](https://stackoverflow.com/questions/45649244/google-cloud-dataflow-apache-beam-can-i-use-sideinputs-with-textio-write) if TextSink supported ValueProviders for headers/footers.

Imported from Jira [BEAM-2771](https://issues.apache.org/jira/browse/BEAM-2771). Original Jira may contain additional context.
Reported by: lcwik.

Start by reviewing Apache Beam's ValueProvider and side-input concepts, then inspect the TextSink/TextIO write entry point mentioned in the issue. Use the linked Jira ticket and Stack Overflow discussion for context. Done should mean dynamically materialized side inputs can be used in the same places as ValueProviders, including the cited headers and footers case.

Written by the indexing model from the issue text.