feat: set permissions and member field optional in custom-role sub-module

[imrannayer]

Fix #194

[bharathkkb]

/gcbrun

[imrannayer]

@imrannayer - PTAL at my comment if its reasonable.

@g-awmalik cross variable validation is not supported in Terraform yet. I can add a check locally and only allow resource creation when either one of them is non-empty list but that will not make it clear to the user why resources are not creating.

[g-awmalik]

I see. We can let go this for now but it'll be nice if this can be made into an object instead with two attributes. That way we can check if at least one attribute is set.

[imrannayer]

@g-awmalik can you plz merge. I dont have permission on this repo.