DVPL-3363: splunklib.searchcommands | Provide access to the contents of the file located at `SearchCommand.input_headers['infoPath']`
DVPL-3364: splunklib.searchcommands | Replace MessagesHeader data structure borrowed from Intersplunk.py

+ Unit tests are include with this commit
+ Verified for completeness and correctness in Splunk with ad-hoc integration testing

Signed-off-by: David Noble <dnoble@splunk.com>