Adds support to specify priority and allow for template override

[wwadge]

Allows specifying priority eg

SpringCloudContractRestDocs.dslContract(Maps.of("priority", 1))))

and makes it possible to override the dsl template

 new ContractDslSnippet(){
                            @Override
                            protected String getTemplate() {
                                return "custom-dsl-contract";
                            }
                        }));

(template must live in resources/org/springframework/restdocs/templates)

Fixes gh-1818

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[marcingrzejszczak]

Thanks @wwadge for your PR! Actually one more question, can you also add a PR with docs update that will mention this feature?

[wwadge]

Sure - let me see if I can find the right docs to update and open a new PR

[wwadge]

Docs here: #1883