Add support for file-based decision logger

[tsandall]

OPA currently supports 3 options for decision logging:

• HTTP-based batched uploads of decision logs via the Decision Log API
• Console-based streaming decision logs
• Plugin/hook-based handling of decision logs (handles recompile)

OPA does not currently support a file-based decision logger that writes decision logs to disk. In theory, OPA could be extended with another option that allows decision logs to written to a file. The logger should support rotation by allowing users to configure how often (based on file size, # of decision log records, or time) the files are rotated.

[stale]

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