introduce getPeerCertificates, fixes #13299 (#13650)

* make i2d_X509 and d2i_X509 always available

i2d_X509 and d2i_X509 have been available in all versions of OpenSSL, so
make them available even if nimDisableCertificateValidation is set.

* introduce getPeerCertificates, fixes #13299

getPeerCertificates retrieves the verified certificate chain of the peer
we are connected to through an SSL-wrapped Socket/AsyncSocket. This
introduces the new type Certificate which stores a DER-encoded X509 certificate.

lib/pure/asyncnet.nim

@@ -95,6 +95,8 @@
 ##   runForever()
 ##
 
+include "system/inclrtl"
+
 import asyncdispatch
 import nativesockets
 import net
@@ -743,6 +745,17 @@ when defineSsl:
     of handshakeAsServer:
       sslSetAcceptState(socket.sslHandle)
 
+  proc getPeerCertificates*(socket: AsyncSocket): seq[Certificate] {.since: (1, 1).} =
+    ## Returns the certificate chain received by the peer we are connected to
+    ## through the given socket.
+    ## The handshake must have been completed and the certificate chain must
+    ## have been verified successfully or else an empty sequence is returned.
+    ## The chain is ordered from leaf certificate to root certificate.
+    if not socket.isSsl:
+      result = newSeq[Certificate]()
+    else:
+      result = getPeerCertificates(socket.sslHandle)
+
 proc getSockOpt*(socket: AsyncSocket, opt: SOBool, level = SOL_SOCKET): bool {.
   tags: [ReadIOEffect].} =
   ## Retrieves option ``opt`` as a boolean value.