gnutls: use session cache for QUIC

Add session reuse for QUIC transfers using GnuTLS. This does not include
support for TLS early data, yet.

Fix check of early data support in common GnuTLS init code to not access
the filter context, as the struct varies between TCP and QUIC
connections.

Closes #15265

tests/http/test_02_download.py

@@ -625,10 +625,16 @@ class TestDownload:
         self.check_downloads(client, srcfile, count)
         # check that TLS earlydata worked as expected
         earlydata = {}
+        reused_session = False
         for line in r.trace_lines:
             m = re.match(r'^\[t-(\d+)] EarlyData: (\d+)', line)
             if m:
                 earlydata[int(m.group(1))] = int(m.group(2))
+                continue
+            m = re.match(r'\[1-1] \* SSL reusing session.*', line)
+            if m:
+                reused_session = True
+        assert reused_session, 'session was not reused for 2nd transfer'
         assert earlydata[0] == 0, f'{earlydata}'
         if proto == 'http/1.1':
             assert earlydata[1] == 69, f'{earlydata}'