Persist downloaded bundle bytes to disk

Earlier with bundle persistence enabled, the bundle
plugin would save the bundle object to disk. In
scenarios where the downloaded bundle has multiple
data files, OPA would first read the bundle and merge
data in the bundle under the bundle.Bundle struct's
Data field. Then before persisting the bundle to disk,
the bundle plugin would use the bundle writer to write
the bundle to the provided output stream. The result
of this is that all the data files in the original
bundle are consolidated into one data.json file.

Now if signature verification is enabled, it will fail
since the files includes in the bundle's signature will not
match the ones in the persisted bundle.

This commit resolves this issue by persiting the bytes
of downloaded bundle to disk which then loaded
from disk maintain the same structure as the original.

Fixes: #3472

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>