Refactorings regarding dcap

[Niederb]

After we introduced it makes sense to do some additional refactorings in order to make the API clearer. Separated this into its own issue to not further complicate things regarding merging. Todo:

• Rename register_enclave to register_ias_enclave
• Use the Fmspc type definition from teerex-primitives also in remote_attestation.rs in the worker as it is duplicated there

[brenzi]

also change the extrinsic name in the worker as this is a breaking change