posix: cond: abstract pthread_cond_t as uint32_t

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

This change deprecates `PTHREAD_COND_DEFINE()` in favour of the
(standardized) `PTHREAD_COND_INITIALIZER`.

This change introduces `CONFIG_MAX_PTHREAD_COND_COUNT`.

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