fix(identity)!: update type of private key

[cedoor]

Description

This fixes #799 by updating the @zk-kit/eddsa-poseidon dependency and adds two methods to export/import encoded (base64) private keys.

The type of the private key in the Identity constructor has also been changed. Now, it must be one of the following: Uint8Array, Buffer, string, i.e. bytes or text, exactly the same as the types supported by @zk-kit/eddsa-poseidon.

This should eliminate the ambiguity of having a string that can be either text or hexadecimal, as it was before.

Related Issue(s)

Fixes #799

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my 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 run yarn format and yarn lint without getting any errors
• 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

[0xjei]

amazing

[artwyman]

Functionality looks good. I have some suggestions for additional testing.