Add legacy SigningCertificate with IssuerSerial for XAdES interoperability

[zipus]

Closes #281

[kislyuk]

Thanks, this looks great!

FYI: SHA1 is deprecated for digital signatures due to cryptographic weaknesses. I'm totally on board with this change, but I will eventually make this code use a different digest algorithm by default; using SHA1 will require passing a custom setting. I will let you know here when I get around to that. For now, this is good.

[kislyuk]

Hmm it looks like there are some CI failures, I'll have to take a look.

[zipus]

Hmm it looks like there are some CI failures, I'll have to take a look.

Ok, let me know if u need anything from my side.

[zipus]

Hmm it looks like there are some CI failures, I'll have to take a look.

While developing on my end other modules depending on signxml I found tests failing due to some certs not having COUNTRY_NAME, ORGANIZATION_NAME, ORGANIZATIONAL_UNIT_NAME, COMMON_NAME. Might be that?

[kislyuk]

The changes look correct, I'll adjust the tests in master. Merging