Do not add validation rules for optional attributes

[erlendnils1]

Optional xsd attributes will currently generate protovalidate rules as if they are mandatory.

For instance:
<attribute name="srsDimension" type="positiveInteger"/>

generates the following proto definition:

uint32 srs_dimension = 22 [
  (buf.validate.field).uint32.gt = 0
];

As xsd attributes are default optional the xml will allow omitting srsDimension. The proto schema will, however, require a positive integer value. It should allow omitting the field / specifying 0.

TODO:
How should this be handled? Current impl simply ignores optional attributes. We could probably create separate rule mappings for required/non required, ie mapping "postiveInteger"+required -> (buf.validate.field).uint32.gt = 0 and "postiveInteger"+non required -> (buf.validate.field).uint32.gte = 0 .

The validation annotation generation does, however, seem quite unfinished in it's current state. For elements it is completely disabled (by if (false && configuration.includeValidationRules) { // TODO SKIP FOR NOW AS RULES ARE NOT COMPILING)