Adds a convenience function for inspecting the coroutine frame of a given
seastar task.
Short example of extracting a coroutine argument:
```
(gdb) p *$coro_frame(seastar::local_engine->_current_task)
$1 = {
__resume_fn = 0x2485f80 <sstables::parse(schema const&, sstables::sstable_version_types, sstables::random_access_reader&, sstables::statistics&)>,
...
PointerType_7 = 0x601008e67880,
...
__coro_index = 0 '\000'
...
(gdb) p $downcast_vptr($->PointerType_7)
$2 = (schema *) 0x601008e67880
```
Closesscylladb/scylladb#19479
fdd8b03d4b