[SANTUARIO-615] xades extension implementation

[jrihtarsic]

The purpose of this PR is to implement XAdES support using the pre/post‑processing mechanism for XML signatures introduced in the previous PR 616. This work addresses the XAdES requirements described in SANTUARIO‑402 and SANTUARIO‑416, and currently covers the Basic Signatures profile.

For composing the XAdES structures, the implementation uses JAXB‑generated classes based on the XAdES schema versions 1.4.1 and 1.3.2, combined with Santuario’s internal ElementProxy model.

[jrihtarsic]

Hi @coheigea

Is it possible
To review the following two PRs regarding XAdES-B
First this one:
#616 - adds the feature to be able to implement pre/post processors to the signature

And then this:
#617 - This PR builds on the feature introduced in PR 616 and provides an implementation of the XAdES‑B profile.

In the second M 617 R I also added a tool
src/test/java/org/apache/xml/security/testutils/SelfSignedCertGenerator.java

which generates simple test certificates on the fly so that there is no need to add new keystore/truststore binaries for new key types. The tool is intentionally minimal, has no external dependencies, and is reusable for future work, including for the future contributions on Post‑Quantum Cryptography.
I created it because Mr. @seanjmullan previously raised concerns when I added keystores for the X25519/X448 and EdDSA use-cases, and this now serves as the alternative.