admin: add oom crash type to /trigger_crash

Allocate memory in a loop until the seastar allocator fails: ducktape
tests always run with --abort-on-seastar-bad-alloc, so the failing
allocation dumps the seastar memory diagnostics and aborts the process,
reproducing what happens when redpanda runs out of memory in a test.
Chunks are kept small to avoid oversized allocation warnings.

With the system allocator (e.g. debug builds) there is no seastar
memory limit and unbounded allocation would exhaust host memory, so the
endpoint refuses the request in that case.