Turn polkit-agent support into an optional feature.

This simplifies deployment on embedded devices, where polkit is usually
unncessary at runtime, but pulls in quite a few otherwise unncessary
dependencies. Start to improve the situation by allowing to selectively
disable polkit-agent support at compile time, which aids in container
usage scenarios, where one wants to invoke 'sysprof-cli' from within
the container. Bypassing polkit-agent in the container is then desired,
since the host sysprofd will handle asking for permissions to enable
the tracing. It allows for a simpler setup of rootless podman
containers, avoiding UID mismatches, that lead to rejection of the
tracing enablement.

- Add a new 'polkit-agent' meson build feature, that allows to force disabling
  polkit-agent support (-Dpolkit-agent=disabled).

- Mark the 'polkit-agent' feature as enabled, by default, to reflect
  the current status (sysprof-cli did not build without polkit-agent support).

- libsysprof/sysprof-instrument.c: Build fix when polkit is not available,
  remove the unnecessary 'g_autopr(PolkitDetails) details' variable.

- Alter the sysprof-cli dependencies to only attempt to link against
  polkit-agent, if necessary. Modify sysprof-cli.c to wrap all code using
  polkit-agent in HAVE_POLKIT_AGENT blocks.

src/libsysprof/sysprof-instrument.c

@@ -178,7 +178,6 @@ _sysprof_instruments_acquire_policy (GPtrArray        *instruments,
                                      SysprofRecording *recording)
 {
   g_autoptr(GDBusConnection) connection = NULL;
-  g_autoptr(PolkitDetails) details = NULL;
   g_autoptr(GError) error = NULL;
   g_auto(GStrv) required_policy = NULL;
 
@@ -202,7 +201,7 @@ _sysprof_instruments_acquire_policy (GPtrArray        *instruments,
         {
           if (!dex_await_boolean (_sysprof_polkit_authorize (connection,
                                                              required_policy[i],
-                                                             details,
+                                                             NULL,
                                                              TRUE), &error))
             return dex_future_new_for_error (g_steal_pointer (&error));
         }