crypto: expose certificate decoding function

Format is the same as:

• https://nodejs.org/api/tls.html#tls_tlssocket_getcertificate

No docs or tests yet, @nodejs/crypto, I'll finish this if we want it.

Its easy, it just exposes current data format of the tls APIs, and makes testing whether certificates can be decoded quite easy, rather than having to round-trip them through TLS just to get a parsed cert :-(.

Maybe some other format would be better, and then it could be added to tls and crypto, but starting with a "better" format in crypto that is different from what tls does seems like it would increase inconsistency.

Fixes: https://github.com/nodejs/node/issues/29181

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines