kernel: add `k_heap_array_get`

Add `k_heap_array_get` as an alternative to `sys_heap_array_get`, which
only returns statically defined heaps (those defined with
`K_HEAP_DEFINE` or `K_HEAP_DEFINE_NOCACHE`), but doesn't depend on the
application guessing a value for `CONFIG_SYS_HEAP_ARRAY_SIZE`.

Signed-off-by: Jordan Yates <jordan@embeint.com>