Identity request

[orhoj]

Purpose

Add function for generating an identity request.

Changes

• JNI for accessing concordium-base functionality for creating an identity request.
• Improved typing of PRF-key and IdCredSec on ConcordiumHdWallet.

Note: The identity request is just given as a String as that is all that is needed for sending it to the identity provider. In a later PR we may type it instead (we have another branch with the majority of the typing done).

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.