Fix #678: allow configuring custom SerializerFactory via JacksonXmlModule

[cowtowncoder]

Implements a feature to address #678.

[cowtowncoder]

@SimonCockx Ok so I don't think there is (alas!) any way to fix obvious handling to allow direct setting via ObjectMapper -- but it seems straight-forward to add functionality to pass SerializerFactory as module configuration like so:

ObjectMapper mapper = XmlMapper.builder()
    .addModule(new JacksonXmlModule().overrideSerializerFactory(RosettaSerialiserFactory.INSTANCE))
    .build();

which should then set SerializerFactory before module registration.

This PR is for adding that functionality, although in the meantime your work-around seems legit (if non-obvious).

LMK what do you think. Addition would go in 2.18.2 which is some way off as I just released 2.18.1 earlier today.

[cowtowncoder]

I think this makes sense, will merge.

[SimonCockx]

@cowtowncoder I completely missed this, sorry. Thanks for the PR, it looks good!

[cowtowncoder]

No problem, figured there's not much downside so could merge it. But will take a while to be available -- I did add it in 2.18 (minor deviation from SemVer), but since 2.18.1 was just released 2.18.2 won't be immediate, will be a month or two, likely.