FFI: add *_count() functions for some FFI enumerators

[reneme]

This adds functions to query the number of entries in index-based enumerator functions that were introduced recently in #5220 and #5217 as suggested.

[randombit]

I thought about asking about this in the original PR but thought maybe the usage pattern would be to just realloc as you go until you get the no-more-data error code. But knowing the count in advance allows preallocation which probably makes life a bit easier for the C programmers...

[arckoor]

While you're at it, could you add botan_x509_cert_get_issuer_dn_count and botan_x509_cert_get_subject_dn_count? These two were introduced in 2.0, and also have that behaviour, but aren't exposed in botan-rs yet so I still have the chance to use the nicer api there :p

[reneme]

While you're at it, could you add botan_x509_cert_get_issuer_dn_count and botan_x509_cert_get_subject_dn_count?

I'll do a follow-up PR. Edit: #5234.