Design how to access the fields of mbedtls_ecp_keypair and mbedtls_ecdsa_context

[gabor-mezei-arm]

This is a design task, as part of #4683 and #5441, to solve the private field access of mbedtls_ecp_keypair (and inclusively mbedtls_ecdsa_context) by MBEDTLS_PRIVATE macro in programs.

Possible option to solve the private access:

1. Make the private fields public
   • good for inner structures
   • locks the representation of the built-in typed data
2. Create getter and setter APIs for field access
   • locks the representation of the built-in typed data
   • requires more memory and more CPU time
3. Some mixture of the previous two option. Some fields are public, some are using getter and setter APIs

The advantages and downsides of this problem explained in detail here.

This is intended to be the design task and the implementation should be done in #4838.

[paul-elliott-arm]

ECP Keypair export function added in #5642 as per discussion in #4838