Move pydantic message classes to separate file

[kumaranvpl]

Description

Type of change

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a fix or feature that would disrupt existing functionality)
• This change requires a documentation update

Checklist

• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• I have included code examples to illustrate the modifications

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
fastagency/adapters/fastapi/base.py	39.25% <ø> (ø)
fastagency/adapters/nats/base.py	18.88% <ø> (-3.87%)	⬇️
fastagency/messages.py	82.78% <100.00%> (-1.73%)	⬇️

... and 4 files with indirect coverage changes