feat: add stronghold_ext_storage for Es256 support

[nanderstabel]

Description of change

See title.

Links to any relevant issues

• Support DIIP V2.1 specifications identity-wallet#177

How the change has been tested

This change has been tested through UniMe and UniCore:

• feat: add support for ES256 identity-wallet#236
• feat: add support for ES256 ssi-agent#57

Disclaimer: for some reason some of the existing tests take over a minute to execute (successfully). I have a suspicion that this has something to do with the stronghold work factor but I have not been able to fix this. For this reason I have not yet added unit tests to prove that the added functionality works.
Interestingly, using this new and existing code in UniMe and UniCore does not seem to affect the performance.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for 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
• 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