Add multi-line support in file collector

If a log message is multi-line in the monitored file, file collector will send events for each line. We should have a regex to define line format (including multi-line support).

Start by locating the file collector and reading how it currently turns lines from a monitored file into events. Define the expected regex behavior for single-line and multi-line log messages, then verify that a multi-line message produces one event rather than one event per line. No specific source file or test is named in the issue.

Written by the indexing model from the issue text.