feat: fetch root key before making calls

[krpeacock]

Description

Since the root key cannot be fetched inside a synchronous constructor, we should await the root key when the shouldFetchRootKey flag is set.

This PR adds a guard to the HttpAgent async calls to make sure the root key call has finished before making the call. It also modifies some checks for rootKey in the canisterStatus and actor utils that should instead wait for the call to be completed.

This shores up occasional inconsistencies with the agent during local development, and that have affected projects in our examples repostory

Fixes # SDK-1905

How Has This Been Tested?

New tests added to the HttpAgent unit test suite

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	72.38 KB (+0.41% 🔺)
@dfinity/candid	11.65 KB (0%)
@dfinity/principal	4.16 KB (0%)
@dfinity/auth-client	50.33 KB (+0.49% 🔺)
@dfinity/assets	67.92 KB (+0.1% 🔺)
@dfinity/identity	47.94 KB (+0.42% 🔺)
@dfinity/identity-secp256k1	106.42 KB (+0.1% 🔺)

[krpeacock]

There is one arguably breaking change - the rootKey is now typed as ArrayBuffer | null. However, I'd say that this typing makes the reality of the workflow explicit. In the past, we would always set the rootKey to the IC_ROOT_KEY regardless of whether we were going to fetch one or not, leading to the potential asynchronous errors we've already encountered.

This does mean that according to TypeScript, the HttpAgent exported in this version wouldn't be usable in an Actor from version 2.0.0. However, we generally see the reverse pattern in practice. Generally people use an up-to-date Actor with an outdated agent.

I think this can be shipped as a minor version update, for this reason.