Introduce mbedtls_pk_get_psa_attributes

Follow the specification in https://github.com/Mbed-TLS/mbedtls/pull/8657
as of dd77343381161e09a63b4694001da3957e27d3a7, i.e.
https://github.com/Mbed-TLS/mbedtls/blob/dd77343381161e09a63b4694001da3957e27d3a7/docs/architecture/psa-migration/psa-legacy-bridges.md#api-to-create-a-psa-key-from-a-pk-context

This commit introduces the function declaration, its documentation, the
definition without the interesting parts and a negative unit test function.
Subsequent commits will add RSA, ECC and PK_OPAQUE support.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_pk.data b/tests/suites/test_suite_pk.data
index af1e20c..e8ffff4 100644
--- a/tests/suites/test_suite_pk.data
+++ b/tests/suites/test_suite_pk.data
@@ -680,3 +680,8 @@
 PSA wrapped sign ext: RSA2048, PK_RSASSA_PSS, MD_SHA512
 depends_on:MBEDTLS_PKCS1_V21:MBEDTLS_MD_CAN_SHA512:MBEDTLS_RSA_C
 pk_psa_wrap_sign_ext:MBEDTLS_PK_RSA:2048:MBEDTLS_PK_RSASSA_PSS:MBEDTLS_MD_SHA512
+
+PSA attributes for pk: NONE (bad)
+pk_get_psa_attributes_fail:MBEDTLS_PK_NONE:0:PSA_KEY_USAGE_SIGN_MESSAGE:MBEDTLS_ERR_PK_BAD_INPUT_DATA
+
+# There is a (negative) test for pk_type=MBEDTLS_PK_RSA_ALT in pk_rsa_alt().