Add component-level diagnostics to API

[fearful-symmetry]

Part of elastic/elastic-agent#2141 and elastic/elastic-agent#2140

This PR contains a few changes:

• Adds a level field to the ActionRequest message type that tags the action as acting on either a unit or the whole component. This contains three level types, with the first being ALL in order to make sure that older non-compatible clients will behave in the expected way. Right now, only diagnostics care about this field.
• Alters the behavior of tryPerformDiagnostics to support the level field; diagnostics that are registered at the client-level become associated with the component level, and diagnostics registered with a unit become associated with the unit level.
• Adds the concept of a optional diagnostic via RegisterOptionalDiagnosticHook(). These optional diagnostics are triggered by corresponding tag in the params field of the ActionRequest message.
• Adds an exported DiagnosticParams that allows the diagnostic actions to take advantage of the params field in a standardized way.
• Adds a CPU profile diagnostic tagged as optional.
• Add a number of tests.

Additional things to consider:

• I'm sure if it's worth expanding DiagnosticParams in additional ways while we're here; there's easily a few forward-looking field we could add, but right now this was strictly focused on supporting optional CPU profiling.
• I made the ActionRequest changes with the goal of making changes both non-breaking and relatively consistent with other parts of the API, but I'm not particularly happy with stuff like If level=component, then the consumer should ignore the unit_id and unit_type fields. My other idea was to break apart the ActionRequest message into different types entirely, perhaps with a different API request for diagnostics, as opposed to the currently strategy of using a single API call for actions and diagnostics. That, however, would be a much larger change.

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• Start Time: 2023-07-19T16:29:05.343+0000

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[blakerouse]

Looks good.

Clearly this is backwards compatible change with the ALL for the level, so that is great! I think the splitting of this will provide a clear benefit and will hopefully remove the duplication of some diagnostic information to ever unit when its for the entire component.