Crypto: add an optional entropy to ec_encrypt to achieve idempotency

[bchamagne]

Description

By giving a entropy to the ec_encrypt, the function always return the same value for the same inputs. We'll need this in #1525.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

unit tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules