Add new matcher to check output file content for WordCountIT and WindowedWordCountIT

Test use case like [https://github.com/apache/beam/pull/4755#discussion_r173254182](https://github.com/apache/beam/pull/4755#discussion_r173254182) shows that verify lines of output for integration test makes the validation and debugging easier than simple verify checksum. We'd like to add a new matcher (like FileContentMatcher) to very lines given expected files.

This matcher can be applied to ITs that already use FileChecksumMatcher (etc. WordCountIT and WindowedWordCountIT).

Imported from Jira [BEAM-3810](https://issues.apache.org/jira/browse/BEAM-3810). Original Jira may contain additional context.
Reported by: markflyhigh.

Start by reading the existing FileContentMatcher and FileChecksumMatcher implementations, then inspect WordCountIT and WindowedWordCountIT to see how checksum validation is configured. Review the linked pull request discussion for expected line-by-line behavior. Done means the new matcher validates output lines against expected files and can be used by both integration tests.

Written by the indexing model from the issue text.