Add ability to configure workspace specific defaults for springboot rule

[plaird]

As a user of the springboot rule
I would like to be able to override the rule's defaults for my workspace to suit my preferences
Such that I can force default dupeclassescheck_enable=True (and other features) on all usages within my Bazel workspace

Background:
As rule authors, we decided on defaults for the various features supported by our rule.
Our default of dupeclassescheck_enable = False is the most notable.
We set our defaults to be user friendly for new users; enabling fail on duplicates by default could be an annoyance for new users.
We allow each usage of the rule to override these defaults as desired using the rule attributes.

But it would be nice (as we are seeing in an internal workspace of ours) to be able to globally (within the workspace) override the default. In our particular case, to enable dupe class checking by default.
Other attributes that could benefit from this treatment include deps_exclude, deps_use_starlark_order, tags, visibility.

The right time to do this is when working #30 since this will likely require backwards incompatible changes to the way the rule is loaded in WORKSPACe.