Add JSON array serialization format to S3 output plugin

[vcschapp]

Is your feature request related to a problem? Please describe.

I need to deliver log events to Amazon S3 as a JSON array rather than newline-delimited JSON.

Describe the solution you'd like

1. Add a new format configuration parameter to the Amazon S3 output plugin with supported values: jsonl (default) and json_array.
2. The default /jsonl behavior continues to be writing newline-delimited JSON.
3. In the json_array variant, the plugin writes a leading [ as the first character of the object, uses , as the delimiter instead of newline, and writes a final ] when the object is finished.

Describe alternatives you've considered

I need my objects to contain JSON arrays for compatibility with an existing web service I do not control. My only options are:

1. Don't use FB.
2. Write a new custom plugin specifically for my use case that does what I want.
3. Make the existing Amazon S3 plugin more versatile.

Additional context

• I am trying to send usage logs to an existing web service whose "public interface" is an S3 bucket that MUST receive objects containing JSON arrays.