[v2] Implement lazy plugin initialization

[aemous]

Issue #, if available:

• Contributes to AWS CLI slow startup #9011.

Notes:

• This PR (4/4) is part of an ongoing effort to reduce AWS CLI initialization time by lazily deferring plugins until they are needed. This PR uses the constructs introduced in previous PRs to lazily import plugins when they are needed (event emission time, command invocation time, and/or help docs generation time).

Description of changes:

• Switch from using HierarchicalEmitter to LazyInitEmitter to facilitate lazy plugin loading.
• Delete handlers.py and load plugin registry from plugin.py.
• Update exe/pyinstaller/hook-awscli.py, to dynamically import builtin plugin modules from the plugin registry.
• Update a few functional/unit tests that previously used register() against events, to use register_last() instead. This is required for the tests to pass, because previously the tests would register after the builtin handlers in botocore, but since switching to lazy initialization, the test code would run before the boto handlers are executed.
• Add tests that improve test coverage on the lazy plugin initialization components.
• Add clarifying comments to tests/unit/test_lazy_emitter.py.
• Add changelog entry.

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