X509 to DER encoding

Hello,

and thank you for the work on this library so far.

I'm having some issues when trying to extract the DER encoding from a X509Certificate. Using this library together with OpenSSL_jll we have access to `i2d_x509` which does this conversion. However, it produces a segmentation fault.

```julia
using OpenSSL, OpenSSL_jll

function der(cert::X509Certificate)
buf = Vector{UInt8}(undef, 2048)
len = @ccall libssl.i2d_X509(cert::X509Certificate, buf::Ptr{UInt8})::Cint
@info len
end

# ------------ output
# Segmentation fault: 11 in expression starting at REPL[10]:1
# ASN1_put_object at /lib/libcrypto.3.dylib (unknown line)
# ASN1_item_ex_i2d at /lib/libcrypto.3.dylib (unknown line)
# Allocations: 660735 (Pool: 660175; Big: 560); GC: 1
```

As noted in other issues, it seems I/O is somewhat problematic currently. But maybe I'm doing something wrong (I also tried `i2d_x509_bio` with the BIO struct provided by this library, but it also produces errors like this).

Thank you for any help.