PK: move the opaque context to a new member of the mbedtls_pk_context structure

[valeriosetti]

With the final goal to share as much code as possible between the following topics:

• improvements in PK module for opaque keys: PK: add support for private key writing with "opaque" EC keys #7484, PK: add support for check_pair() with "opaque" EC keys #7485
• new management of public and private key materials: Avoid parse/unparse public ECC keys in PK with USE_PSA when !ECP_C #7073, Avoid parse/unparse private ECC keys in PK with USE_PSA #7074

the purpose of this task is to:

• introduce a new priv_id field inside the mbedtls_pk_context structure
• move existing PK_OPAQUE to use it instead of pk_ctx