[chore] [cmd/schemagen] Schemagen script improvements

[jkoronaAtCisco]

Description

Few improvements added to schemagen script:

1. Removed obsolete fields from the schema
2. Internal dependencies are not embedded within schema but rather referenced with relative url
3. Shorter time.Duration declarations, recognized by format.
4. Optimized schemas validation
5. Use snake_case instead of PascalCase for sub-schema names

Link to tracking issue

#42214

Testing

• Unit tests
• See icmpcheckreceiver component as an example of updated schemas

Documentation

• README.md

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