feat: Add fetchCandid() function to @dfinity/agent

[rvanasa]

Description

This PR adds a function to abstract away the implementation details for retrieving the current Candid interface based on canister metadata and the hidden __get_candid_interface_tmp_hack method (for canisters without this metadata).

At the moment, several projects (notably the IC Dashboard and Candid UI) directly call the _tmp_hack method. This fetchCandid() function can be used as a replacement in both of these web applications.

Example usage:

import { fetchCandid, HttpAgent } from '@dfinity/agent';

const agent = new HttpAgent(...);

const candidSource = await fetchCandid(agent, 'ryjl3-tyaaa-aaaaa-aaaba-cai');

How Has This Been Tested?

• Added a new test case: agent/src/fetch_candid.test.ts
• Works as expected in a browser environment.

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. (JSDoc)

[krpeacock]

I think this is more intuitive than CanisterStatus, so it's a good idea! One suggestion - since candid is public metadata, why not move agent to the second position and make it optional, using an anonymous HttpAgent by default?

edit - pinging @rvanasa

[rvanasa]

Sounds good; updated to use the same anonymous HttpAgent logic from both Candid UI and Motoko Playground.

I also added HttpAgent::isLocal() to simplify the relatively common use case of fetching the root key for local environments. Let me know if there's a better way of approaching this from within the agent-js codebase.

[krpeacock]

We've avoided the concept of local or mainnet for the sake of handling testnets more naturally for NNS dapps, but I don't think I mind introducing this pattern now - it's useful for most use cases