refactor: app design concept

[cachho]

Description

Cleans up the code by refactoring the whole AppConfig and App design concept. Introduces separate configs, that make it clear where defaults are used and what's configurable.

This is a major change, posting so that you can review it while I build changes based on it.

Documentation update not included.

This should work as it is, but it is a breaking change because InitConfig disappears in favor of AppConfig, OpenSourceAppConfig and CustomAppConfig.

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit Test
• Manual Test (App)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Made sure Checks passed

[cachho]

Further refactor by moving App classes out of embedchain.py into Apps folder. This is a true refactor that's not breaking anything.