feat: introduces partial identity

[krpeacock]

Description

To get a delegation from Internet Identity, the Auth Client allowed the developer to initialize the client using an existing identity. However, since the delegation can target any public key, it can be useful to allow the client to accept a "partial" identity class without a private key.

Introduces the PartialIdentity and PartialDelegationIdentity classes in @dfinity/identity, which can now be passed into AuthClient.create

This will be useful to remove the limitations such as adding an intermediary identity, as used by our deep link example: https://github.com/dfinity/examples/blob/c62de1a928fa0bf6b8fdbbbf0e0204e7cf5d6dfb/native-apps/unity_ii_applink/ii_integration_dapp/src/greet_frontend/src/index.js#L49

How Has This Been Tested?

New Unit tests

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	99.52 KB (0%)
@dfinity/candid	13.56 KB (0%)
@dfinity/principal	5.02 KB (0%)
@dfinity/auth-client	91.12 KB (+0.35% 🔺)
@dfinity/assets	91.7 KB (0%)
@dfinity/identity	88.63 KB (+0.23% 🔺)
@dfinity/identity-secp256k1	255.47 KB (0%)