posix: key: abstract pthread_key_t as uint32_t

Consistent with the change of `pthread_t`, `pthread_mutex_t`,
and `pthread_cond_t` to `uint32_t`, we can now also abstract
`pthread_key_t` as `uint32_t` and separate the implementation
detail, hidden from POSIX API consumers.

This change introduces `CONFIG_MAX_PTHREAD_KEY_COUNT`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>