Add RSA private-key round-trip support to COSE_Key (PS256/384/512)

[aidangarske]

Summary

wolfCOSE's COSE_Key serialization currently supports RSA public keys only.
A generated RSA-PSS key therefore cannot be re-loaded for signing after a
COSE_Key encode/decode round-trip, so keygen -> sign -> verify is not yet
available for PS256/384/512 (ECDSA, EdDSA, and ML-DSA round-trip fully).

Scope

Extend the RSA COSE_Key encode/decode to carry the full private key per
RFC 8230 so a round-tripped RSA key can sign. This is a self-contained
enhancement and should land in its own reviewed PR.

Status

Tracked as a known limitation. In PR #33 the RSA decode keeps the documented
public-key-only behavior (with a TODO referencing this issue), and the
command-line tool test smoke-tests RSA keygen while skipping the RSA
sign/verify round-trip until this is implemented.

[aidangarske]

Correction with accurate detail after investigation:

The blocker is bigger than "import n,e,d". wc_RsaPrivateKeyDecodeRaw (the only raw private import API) requires n, e, d, p, q, and u (qInv) — in a non-RSA_LOW_MEM build it returns BAD_FUNC_ARG when u == NULL. But:

• wc_CoseKey_Encode today emits only n, e, d (RFC 8230 labels -1,-2,-3).
• wc_RsaExportKey provides n, e, d, p, q — not qInv.

So a proper fix must:

1. Extend the COSE_Key RSA encoding to also carry p, q (labels -4, -5) and qInv (-8), and ideally dP, dQ (-6, -7) for a spec-complete RFC 8230 private key.
2. Obtain qInv — either read RsaKey->u at encode, or compute qInv = q^-1 mod p (pulls bignum/mp_invmod into the otherwise marshal-only, zero-allocation wolfCOSE core).
3. Decode the new labels and import via wc_RsaPrivateKeyDecodeRaw, with unit + forced-failure coverage.

This is a focused crypto-format change that deserves its own reviewed PR, separate from the ML-DSA rename / CI work in #33. In #33 the decode keeps the documented public-key-only behavior (with a TODO(#34) in wc_CoseKey_Decode), and the new command-line tool test SKIPs RSA-PSS round-trip while still smoke-testing RSA keygen.