feat: introduces Observable Log for HttpAgent

[krpeacock]

Description

The agent currently outputs console logs, warnings, and errors to the console.

This introduces an Observer pattern, available as a public ["log"] attribute which returns a new type of AgentLog. Developers can add listeners to hook into the logs, and a new flag (logToConsole) is provided, which will output these logs to the console.

This is turned off by default.

How Has This Been Tested?

new unit tests

Checklist:

• [x] My changes follow the guidelines in CONTRIBUTING.md.
• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [x] I have made corresponding changes to the documentation.

[github-actions[bot]]

size-limit report 📦

Path	Size
@dfinity/agent	100.18 KB (+0.31% 🔺)
@dfinity/candid	13.61 KB (0%)
@dfinity/principal	5.04 KB (0%)
@dfinity/auth-client	91.79 KB (+0.07% 🔺)
@dfinity/assets	91.96 KB (+0.03% 🔺)
@dfinity/identity	89.09 KB (+0.05% 🔺)
@dfinity/identity-secp256k1	263.22 KB (+0.14% 🔺)