[v2] Lazily import all built-in plugins

[aemous]

Description of changes:

PrefixTrie

• Made PrefixTrie in vendored botocore public.
• Created unit tests for PrefixTrie.

LazyCommand

• Implemented LazyCommand for deferred plugin importing and invocation until the command is invoked or its help docs are generated.
• Implement unit tests and functional tests for LazyCommand.

LazyInitEmitter

• Implemented LazyInitEmitter, which loads a plugin registry to lazily initialize plugins, and automatically wrap customizations in LazyCommands.
• Created unit tests for LazyInitEmitter.

Auto-generated plugin registry

• Created new script to auto-generate plugins registry, and used it to migrate handlers.py monolith to a plugin-events registry (script not committed to repo. It's a one-time use though).
• Committed auto-generated plugins registry.
• For customizations that register against building-command-table.main and other events, split their registrations so that a single function contains registrations against building-command-table.main, and another function contains all other registrations.

Lazy loading

• Use LazyInitEmitter and pass it to Session in botocore.
• Updated plugin.py to use LazyInitEmitter as a default event emitter instead of HierarchicalEmitter.
• Update PyInstaller hook so that it uses the plugins registry to dynamically include lazy-loaded plugins in built exexutables.
• Update handlers.py so it no longer imports and registers all plugins. It uses LazyInitEmitter to facilitate lazy loading of all plugins.
• Update existing tests that depended on botocore's builtin event handlers (in awscli/botocore/handlers.py) to be registered before event handlers defined in the tests, to use register_last instead of register, to preserve behavior of the handlers defined in the tests to be invoked after the builtin handlers in botocore.
• Add missing test cases to tests/functional/test_lazy.py.
• Add changelog entry.

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