Add CountingInput Transform to python sdk

[kennknowles]

Similar to java sdk, https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/CountingInput.java

Imported from Jira BEAM-1069. Original Jira may contain additional context.
Reported by: vikasrk.

[ritikBhandari]

Hi,
I'd like to work on this issue.

[mohamedawnallah]

.take-issue

[mohamedawnallah]

Freeing up my assignment to this good-first-issue so other contributors can jump in if interested! If no one picks it up by then, I'm happy to submit a PR to close it out