fish: enable the legacy provider if build against OpenSSL3

OpenSSL 3.0 disables a number of "legacy" algorithms by default, and we
need to enable them manually using their provider system. Note that
explicitly loading a provider will disable the implicit default
provider, which is why we need to load it explicitly.

Closes #2629

Signed-off-by: Simon Chopin <simon.chopin@canonical.com>

V2:
  * use a local OSSL_LIB_CTX to avoid leaking the legacy algorithms
    into the main SSL context.
  * Simplify the fish_init() error paths by calling fish_deinit()

plugins/fishlim/tests/tests.c

@@ -278,5 +278,8 @@ main(int argc, char *argv[]) {
     g_test_add_func("/fishlim/max_text_command_len", test_max_text_command_len);
     g_test_add_func("/fishlim/foreach_utf8_data_chunks", test_foreach_utf8_data_chunks);
 
-    return g_test_run();
+    fish_init();
+    int ret = g_test_run();
+    fish_deinit();
+    return ret;
 }