Add/update a GitHub action to verify that generated manifests/resources match what is in git

[jgwest]

Add a GitHub action (or add a check to an existing GH action's build steps, if this makes more sense) to verify that generated manifests/resources match what is in git.

Essentially, steps something like this:

# clone the applicationset repo
mkdir /tmp/pre-make
cp -r ./ /tmp/pre-make

# run generator
make manifests

# compare old and new, and fail if different
diff -r manifests /tmp/pre-make
# check for non-zero error code to indicate that the generated files have changed, then fail the build and output a message saying user should run make manifests and check in the results into their PR

This will catch any uncommitted changes to manifests that the user has made, and which require updated artifacts to be checked in for (CRDs, generated .go files).

For example, this would have caught the regression fixed by this PR.

This should also include resources generated by controller-gen.

[jgwest]

The fix for this was included as part of #105