This commit is preparation for next commit with timer using RosTime clock.

In the previous implementation the function rcl_timer_init is called
with zero initialized timer handle being stored on stack and after
initialization with rcl_timer_init the timer handle is moved by rust
inside Timer_ struct.

The problem occurs if the type of clock being passed to rcl_timer_init()
is RosTime because the function also registers callbacks in the clock
that would notify the timer about sudden time change (jump callback).
In RCL the jump callback is registered with rcl_clock_add_jump_callback()
and the argument callback.user_data is pointer to timer handle that in
our case references timer stored om stack. When returning from function
create_wall_timer() the pointer becomes dangling and therefore when the
callback is actually called (by function _rcl_timer_time_jump) it could
result in SIGSEGV or could access some variable that happens to be in
the same place as the timer_handle was on stack.

This is fixed by storing timer_handle inside pinned box to prevent
moving the handle.

This allows to implement timers that can share single clock among them.

This commit is preparation for feature sim time.

This adds Node::create_timer() method. The method is similar to
already existing Node::create_wall_timer(), but the timer is based on
ClockType::RosTime rather than on ClockType::SteadyTime.

The advantage of ClockType::RosTime is that one can drive the clock
from a different time source. This will be used in the next commits to
implement for clock source on the /clock topic.

Added feature flag sim-time because functions that are feature gated does
not make sense without TimeSource that is implemented in later commit
and needs the feature flag.

The TimeSource subscribes to /clock topic and publishes the time to all
attached clocks (by default only clock used by create_timer is attached).

This module is feature gated because the clock is published in message
rosgraph_msgs::msg::Clock and rosgraph_msgs needs to be specified as
dependency by the end users.

…meter

add `--ros-args -p use_sim_time:=true` to starting command line

cfg(r2r__rosgraph_msgs__msg__Clock) attribute is not propagated to user
code so this macro can be used to intentionally fail compilation if
sim time is not supported.