How to sign programmatically

OpenSSL 3.0 provides the following APIs for digital signatures:

• Legacy low-level APIs that consist of functions with algorithm-specific prefixes, such as RSA_, DSA_, and ECDSA_. These APIs have been deprecated since OpenSSL 3.0.
• The EVP_PKEY API: This API is not deprecated but is still low level. It is more convenient to use a high-level API.
• The EVP_Sign API: This API is high level but has some disadvantages. This API uses the key argument only after the whole input data has been read and hashed. Therefore, if there is a problem with the key, it will be discovered later rather than sooner. Another disadvantage is that this API is inflexible and does not allow you to set signing parameters to PKEY_CTX if the signature algorithm supports them. The OpenSSL documentation does not recommend this API.
• The EVP_DigestSign API: This is a high-level API where drawbacks of the EVP_Sign API have been fixed. The OpenSSL documentation recommends...