Feature branch request for EVP_SKEY implementation

[beldmit]

According to https://openssl-library.org/policies/general/feature-branch-approval-policy/ I request a feature branch to implement usage of opaque objects as symmetric keys.

Proposed name: feature/evp_skey
A link to an issue describing the requested feature: openssl/openssl#21573 is a design document I'm going to implement.
ETA: 2025Q1

CC: @simo5, @neverpanic

[beldmit]

@openssl/corporation-members I want to draw you attention that the technical policy https://openssl-library.org/policies/general/feature-branch-approval-policy/ relies on OMC and so needs to be updated.