feat: add delegation identity

[flyq]

Description

Added DelegationIdentity

Fixes # (issue)

https://github.com/dfinity/agent-rs/issues/347

How Has This Been Tested?

I new a DelegationIdentity, and call the canister on chain, it works

Checklist:

• [ ] The title of this PR complies with Conventional Commits.
• [ ] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.

[dfinity-droid-prod[bot]]

Dear @flyq,

In order to potentially merge your code in this open-source repository and therefore proceed with your contribution, we need to have your approval on DFINITY's CLA[^1].

If you decide to agree with it, please visit this issue and read the instructions there.

— The DFINITY Foundation [^1]: Contributor License Agreement

[dfinity-droid-prod[bot]]

Dear @flyq,

In order to potentially merge your code in this open-source repository and therefore proceed with your contribution, we need to have your approval on DFINITY's CLA[^1].

If you decide to agree with it, please visit this issue and read the instructions there.

— The DFINITY Foundation [^1]: Contributor License Agreement

[sesi200]

Thanks a lot for your PR! Have you considered adding a few tests to make sure it works as expected?

[flyq]

Thanks a lot for your PR! Have you considered adding a few tests to make sure it works as expected?

You're welcome. I will add tests. I would love to hear any other review suggestions