llext: add bringup, teardown, and bootstrap APIs

llext_bringup() and llext_teardown() are intended to be used to call the
extension's own initialization and cleanup functions, respectively. They
are meant to be called by the developer after loading an extension and
before unloading it. The list of function pointers to be called is
obtained via the new llext_get_fn_table() syscall, so that they are
compatible with user mode.

llext_bootstrap() is intended to be used as the entry point for a thread
created to run an extension, in either user or kernel contexts. It will
call the extension's own initialization functions and then an additional
entry point in the same context (if desired). The same function can also
be called directly in the main thread, if only initialization is
required.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>