Add configuration file input

[p5]

Closes #217

Adds a new input parameter to allow the user to specify the location of a config file.
This PR includes some tests to ensure the correct flags are passed into Grype, and updates the README.md.

I am unsure what else to add to the tests, or how to actually test that parameters in the config files are working. Hopefully I can have some guidance here.

I have chosen the same naming convention as the Syft action.

[kzantow]

Hey @p5, sorry for the delay here. Is this still something you are interested in getting merged? I think to get this merged it would be useful to have a test validating the CLI option (like these).

However, I should also note that today there are already a couple other ways to configure grype-specific parameters in the action using environment variables, a couple examples are passing a configuration file using environment variables:

      - uses: anchore/scan-action@main
        with:
          image: alpine:3.15
        env:
          GRYPE_CONFIG: ./my-config.yml

... or simply setting specific parameters using the grype environment variables:

      - uses: anchore/scan-action@main
        with:
          image: alpine:3.15
        env:
          GRYPE_ONLY_FIXED: true

... would either of these suffice or would you prefer adding an explicit configuration option?