Config: Add support for elastic features configuration

[SylvainJuge]

• define shared spec for agents configuration
• define compatibility with external configuration files: properties and yaml (not yet implemented but likely already in specs)
• implement it in this agent
• add configuration to any existing feature that might require it:
  • span stack trace added in Capture span stack trace when the span ends #16, duration parsing already provided by ConfigProperties

Implementation notes:

• AutoConfiguredOpenTelemetrySdkBuilder#addPropertiesSupplier adds default values
• AutoConfiguredOpenTelemetrySdk#getConfig provides the actual config

[jackshirazi]

My current experimentation and thought process:

1. We can automate reconciliation of the config declarations by accessing all the configs in the latest agent jar and checking that we have corresponding definitions for them in the new agent. The new agent will have an additional "not yet implemented" declaration for any not yet implemented
2. We don't use the stagemonitor dependency in the new agent (except for that test reconciliation process in 1. above), instead we implement the parts we need - it would look fairly similar though because ...
3. We need to generate the docs in the same format so we may as well copy over the doc generation
4. The new stagemonitor re-implementation needs to check the same 4 things (JVM properties, environment variables, file property declaration, central config)
5. An Otel extension will run the configure process at start and accept values (with an option to be sync/async for the central config?) and set the equivalent Otel values if they exist - this would happen at agent start, there is an extension hook

[jackshirazi]

Check opamp for connecting to central config instead of v2 connections

[jackshirazi]

Added a set of tasks to get to parity in #33 , beyond parity (eg yaml property files, ...) should be opened in new issues