Implement key derivation input getters

[gilles-peskine-arm]

This a step of the implementation of the interface for key derivation drivers introduced in #5451. It follows #5477. The goal of this issue is:

• Introduce accessor functions for psa_crypto_driver_key_derivation_inputs_t: psa_crypto_driver_key_derivation_get_input_size, psa_crypto_driver_key_derivation_get_input_bytes. No psa_crypto_driver_key_derivation_get_input_key yet: that will come with opaque driver support. No psa_crypto_driver_key_derivation_get_input_integer yet: we don't yet implement any KDF with integer parameters.
• Create unit tests for these functions.
• Use these functions instead of direct access in psa_crypto_key_derivation_driver_setup.

[mprse]

From the documentation getter functions should have the following declaration:

psa_status_t psa_crypto_driver_key_derivation_get_input_size(
    const psa_crypto_driver_key_derivation_inputs_t *inputs,
    psa_key_derivation_step_t step,
    size_t *size);

psa_status_t psa_crypto_driver_key_derivation_get_input_bytes(
    const psa_crypto_driver_key_derivation_inputs_t *inputs,
    psa_key_derivation_step_t step,
    uint8_t *buffer, size_t buffer_size, size_t *buffer_length);

I understand that psa_crypto_driver_key_derivation_inputs_t is the union of inputs for different algs (hkdf, prf, ...):

typedef struct
{
    union
    {
        /* Make the union non-empty even with no supported algorithms. */
        uint8_t MBEDTLS_PRIVATE(dummy);
#if defined(MBEDTLS_PSA_BUILTIN_ALG_TLS12_PRF) || \
    defined(MBEDTLS_PSA_BUILTIN_ALG_TLS12_PSK_TO_MS)
        psa_tls12_prf_key_derivation_inputs_t MBEDTLS_PRIVATE(tls12_prf);
#endif /* MBEDTLS_PSA_BUILTIN_ALG_TLS12_PRF ||
          MBEDTLS_PSA_BUILTIN_ALG_TLS12_PSK_TO_MS */
#if defined(MBEDTLS_PSA_BUILTIN_ALG_HKDF)
        psa_hkdf_key_derivation_inputs_t MBEDTLS_PRIVATE(hkdf);
    };
#endif /* MBEDTLS_PSA_BUILTIN_ALG_HKDF */
} psa_crypto_driver_key_derivation_inputs_t;

So we need here to pass also operation to determine the alg and distinguish different input types to process.