feat(agent): load root key from ic_env cookie

[ilbertt]

Description

Load root key from the ic_env cookie if no root key is specified and shouldFetchRootKey is false.

Blocked by #1156.

How Has This Been Tested?

Added 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	50.87 KB (-0.04% 🔽)
@dfinity/assets	55.77 KB (+0.5% 🔺)
@dfinity/auth-client	21.16 KB (0%)
@dfinity/candid	13.3 KB (0%)
@icp-sdk/core	91 B (0%)
@dfinity/identity	20.61 KB (0%)
@dfinity/identity-secp256k1	33.88 KB (0%)
@dfinity/principal	4.44 KB (0%)

[ilbertt]

We decided to not do it automatically in the agent. Closing this PR.