add an option to output the report into a file rather than redirecting the output

[wagde-orca]

What would you like to be added:
I would like to have an option to output the report (json) into a file with an option in the commad line like
grype "dir:/xxx" -o json -f report.json
Why is this needed:
it's easier with this option to integrate grype in python code rather than running and getting the output with subprocess
Additional context:

[wagoodman]

@wagde-orca today we report all status output (e.g. "loading image", "parsing image", etc...) to stderr and all report data to stdout. In this way if you have an integration where you are trying to wrap grype and get the output you can safely grab all of stdout.

Using a redirect with bash:

# this will ignore all stderr status-like output
grype some-image:latest -o json > report.json

Or subprocess in python:

import subprocess
result = subprocess.run(['grype', 'some-image:latest', '-o', 'json'], stdout=subprocess.PIPE)
result.stdout # <--- the full json report

Shout out if you were intending something different or if this doesn't fit your needs.

[wagde-orca]

@wagoodman thanx
This is the way I do it right now and it is not so bad... but i still believe that having an option to dump the output to a file can be helpful

[wagoodman]

Tracking additional context from a duplicate issue #418:

What would you like to be added:
An official docker image for Grype that either has a shell, or an option to output findings to a file.

Why is this needed:
We're using Kubernetes native tooling for CI/CD (Tekton), and there isn't really an option to use Grype's official docker containers, because we need to output to a file, and it only outputs to stdout. Of course, an easy fix would be to just forward STDOUT to a file, but you can't do that in the docker container, because it has no shell.

There was a PR for a solution for this in Tekton but it's been closed and no ETA: tektoncd/pipeline#3103
Similar situation with k8s: kubernetes/kubernetes#94892