feat: migrate to secure ECDSA key for auth-client

[krpeacock]

Description

Now that we have the non-exportable ECDSAKeyIdentity using CryptoKeys, we can use it as the base key for II Delegations.

Fixes # FOLLOW-814

How Has This Been Tested?

New unit tests, also by compiling the package and using it in the auth-client-demo application

Checklist:

• My changes follow the guidelines in CONTRIBUTING.md.
• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[github-actions]

size-limit report 📦

Path	Size
@dfinity/agent	89.89 KB (0%)
@dfinity/candid	15.46 KB (0%)
@dfinity/principal	6.77 KB (0%)
@dfinity/auth-client	95.46 KB (+0.53% 🔺)
@dfinity/assets	92.07 KB (0%)
@dfinity/identity	92.35 KB (0%)
@dfinity/identity-secp256k1	229.99 KB (0%)