extmod/modussl_mbedtls.c: Add ussl.getpeercert() method.

Behaviour is as per CPython but only the binary form is implemented here.
A test is included.

tests/net_hosted/ssl_getpeercert.py

@@ -0,0 +1,21 @@
+# test ssl.getpeercert() method
+
+try:
+    import usocket as socket
+    import ussl as ssl
+except:
+    import socket
+    import ssl
+
+
+def test(peer_addr):
+    s = socket.socket()
+    s.connect(peer_addr)
+    s = ssl.wrap_socket(s)
+    cert = s.getpeercert(True)
+    print(type(cert), len(cert) > 100)
+    s.close()
+
+
+if __name__ == "__main__":
+    test(socket.getaddrinfo('micropython.org', 443)[0][-1])

tests/net_hosted/ssl_getpeercert.py.exp

@@ -0,0 +1 @@
+<class 'bytes'> True