feat: deprecate HttpAgent constructor in favor of new create

[krpeacock]

Description

Following the pattern used by other interfaces in the library, this function switches to an async create method, that will be able to make the syncTime and fetchRootKey calls before completing. This will also support future async setup calls if needed in the future. createSync initializes the agent but skips any async steps, and is useful for testing or more sophisticated applications

The agent also has a from method, and will support either the v1 or v2 agent interfaces.

The HttpAgent also switches to true es6 private class fields, and will formally reveal its config and its host as fields. This supports the from method, but the _-prefixed methods from older HttpAgents will be supported.

Note: the constructor is marked deprecated in this PR. Existing code will not break yet, but will inform the developer that they should switch to create or createSync`.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

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	85.7 KB (+0.1% 🔺)
@dfinity/candid	13.58 KB (0%)
@dfinity/principal	4.97 KB (0%)
@dfinity/auth-client	60.75 KB (+0.07% 🔺)
@dfinity/assets	80.28 KB (+0.05% 🔺)
@dfinity/identity	57.94 KB (+0.05% 🔺)
@dfinity/identity-secp256k1	265.87 KB (+0.09% 🔺)

[krpeacock]

@roelstorms