Add template func hasField, update README

[jlehman9]

• Adds the template function hasField.
  • Allows template to check if a field is defined in a given JSON struct.
• Update README.md to document custom template functions.

This came from an issue I faced trying to write a CSV template function that grabbed vendor/author/publisher data. Unfortunately those fields aren't always defined for every package, and attempting to access them will cause Syft to exit with something like
* template: csv.tmpl:3:55: executing "csv.tmpl" at <.Metadata.Author>: can't evaluate field Author in type interface {}

I figured it would also be good to document the where the functions are defined in the README.md so others can see they exist and offer new ones.

Thanks to @kzantow for the help discovering this solution!

[kzantow]

Hey, thanks @jlehman9 ! would you be able to add a simple test that executes this function with a template?

[jlehman9]

Hey, thanks @jlehman9 ! would you be able to add a simple test that executes this function with a template?

I have no idea what I'm doing since I don't really know Go, but I think I've got tests working that test the hasField function. Let me know if there is anything else I can do/fix/etc.

[kzantow]

It looks like there are quite a few failing tests due to the changes, maybe just make a new test?

Also, you'll need to sign-off your commits.

[kzantow]

LGTM

[jlehman9]

I believe I've undone my previous poor commit, created a separate test, re-signed the bad commit, and ensured all of the tests pass on my local machine. Hopefully all is good?

Previously I had improperly assumed that the failing tests were all due to my local environment, mainly due to not having docker daemon installed since I usually run docker from minikube but the tests couldn't see that. Sorry about that!

[kzantow]

Thanks for the contribution @jlehman9!