Add ability to process xpath expressions during parse

[seadowg]

Work toward getodk/collect#5620

This adds a new XPathProcessor for XFormProcessor that allows a client to inspect every XPath expression in an XForm during parse:

parser.addProcessor(new XFormParser.XPathProcessor() {
    @Override
    public void processXPath(@NotNull XPathExpression xPathExpression) {
        // Do something
    }
});

I've also added a lock to XFormParser#parse that will prevent two forms being parsed at the same time (even using two different XFormParser instances) as XPathProcessor (and existing parse code) manipulates static state that could cause problems if that were to happen.

What has been done to verify that this works as intended?

New tests.

Why is this the best possible solution? Were any other approaches considered?

We had initially considered just using our existing bind processing with XPathExpression#containsFunc to find pulldata, but we realised that we needed a more general solution to be able to detect a custom function anywhere in the form (as it could appear in any xpath expression). I'm not a fan of the static state I've added here, but it felt like a losing battle to avoid it given XFormParser was already not thread safe in any way. Down the line, we should look at improving that so clients like Collect can parse multiple forms in parallel.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Stopping two forms from being parsed simultaneously is probably the riskiest change here. As far as I know, Collect doesn't do this on purpose, but it might be that it's a possibility with things like auto update or match exactly. That said, if that was happening, I'd potentially rather we crash the app than end up with whatever could happen mixing state from two form parses.

[lognaturel]

Took me a bit to understand that this is just so the processors can be accessed statically even though they need to be set on an instance to use the existing addProcessor.