feat: support PID iteration

This change introduces the `lwp_pid_for_each` function, which provides a
convenient and thread-safe method for iterating over PIDs with a user-
defined callback. This addition is necessary to support cases where
operations must be performed on each PID in the balanced tree, enhancing
flexibility and modularity for PID management.

Changes:
- Added `lwp_pid_for_each` function in `lwp_pid.c` to allow iteration
  over PIDs using a callback function and optional data parameter.
- Defined a new internal `pid_foreach_param` structure to encapsulate
  the callback and data for the iteration.
- Added `_before_cb` helper function for executing the callback on each
  PID node during AVL traversal.
- Ensured thread safety by acquiring and releasing the PID lock around
  the AVL traversal within `lwp_pid_for_each`.
- Updated `lwp_pid.h` with the `lwp_pid_for_each` function prototype and
  included `rtthread.h` for necessary types.

Signed-off-by: Shell <[email protected]>

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