Local File Agent: Output event with file pointer when writing to file

[krendil]

Change Local File Agent to also emit events when in write mode. The event emitted contains a file pointer to the local file that was just written or appended to. This makes it more convenient to create workflows that involve writing content to a file and then performing an action on that file.

[krendil]

Some context on the use case behind this PR, in case there is already a better way to accomplish the same thing:

I'm trying to create an agent that will import files to Firefly III, through the Firefly Data Importer. In order to import data this way, you need to make a POST request encoded as multi-part/form-data that includes two different files - the data you are importing as well as a JSON configuration file. As far as I can tell, the POST agent only allows you to create multi-part/form-data requests if the data is already a file pointer (and you can only post one file at a time). Creating a Shell agent to run curl has a similar issue, in that you can only add file sections to the multipart request using data that is saved to a file, using the @filename syntax. Therefore, we need to write the data to a file first.

This can be accomplished with the Local File agent, however the Local File agent is a dead-end in terms of event flow in write mode. If I wanted to do anything with the file, I'd need a second Local File agent that watches for the files created by the first agent. This is an unnecessary complication that has a number of downsides, hence this PR to allow the chain to remain connected.

Before:
(Receive data) → (Write to file)
(Watch files) → (POST to Firefly importer)

After:
(Receive data) → (Write to file) → (POST to Firefly importer)

[dsander]

That makes perfect sense. Thank you for the detailed use case explanation!