Support transactions

[pdavide]

Draft PR for transactions log support.

[LBreda]

Isn't this PR enough to log the request and response as required? Why isn't it merged yet?

[pdavide]

Hi @LBreda, as I wrote in #51, this PR is still a draft, for which the following points are to be developed:

• adding methods to the event for retrieving the information reported on https://docs.italia.it/italia/spid/spid-regole-tecniche/it/stabile/log.html#service-provider (also making sure not to generate unhandled exceptions for any information not in the DOMDocument tree);
• addition of appropriate tests to maintain 100% coverage;
• updating the documentation;

If you would like to contribute to the development, that would be really great.