[fidl] [llcpp] Generate C-binding-like free helper functions for client calls.

Many existing uses of the C-binding client calls are one-off, and do not
assume channel ownership. E.g.

    zx_status_t do_this(zx_handle_t chan, int32_t arg) {
        return fuchsia_do_this(chan, arg, more_args);
    }

This is tricky to refactor into llcpp, because SyncClient owns the
channel. Setting up a SyncClient for every one-off call is also
un-ergonomic.

To faciliate those migrations,
this patch generates additional helper functions, nested under

    fuchsia::lib::Protocol::Call

e.g.

    fuchsia::lib::Protocol::Call::MyMethod(zx::unowned_channel client_end);

and modifies SyncClient to call these instead.

TEST: fidl-llcpp-interop-tests

Change-Id: I595504310433818985bc3876ec682b800c02956a