WIP: Billing service

[zomars]

What does this PR do?

• Fixes #XXXX (GitHub issue number)
• Fixes CAL-XXXX (Linear issue number - should be visible at the bottom of the GitHub issue description)

The goal of this PR is to centralize our internal billing in a shared service. This will allow us to have an overview of all the billing related logic in one place.

• Implements a stripe web hook handler so we can split big API handlers by webhook event, and lazy load them.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected)
• I have added a Docs issue here if this PR makes changes that would require a documentation change
• I have added or modified automated tests that prove my fix is effective or that my feature works (PRs might be rejected if logical changes are not properly tested)

How should this be tested?

• Are there environment variables that should be set?
• What are the minimal test data to have?
• What is expected (happy path) to have (input and output)?
• Any other important info that could help to test that PR

Checklist

• I haven't read the contributing guide
• My code doesn't follow the style guidelines of this project
• I haven't commented my code, particularly in hard-to-understand areas
• I haven't checked if my changes generate no new warnings

[github-actions]

Hey there and thank you for opening this pull request! 👋🏼

We require pull request titles to follow the Conventional Commits specification and it looks like your proposed title needs to be adjusted.

Details:

Unknown release type "WIP" found in pull request title "WIP: Billing service". 

Available types:
 - feat: A new feature
 - fix: A bug fix
 - docs: Documentation only changes
 - style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
 - refactor: A code change that neither fixes a bug nor adds a feature
 - perf: A code change that improves performance
 - test: Adding missing tests or correcting existing tests
 - build: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
 - ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
 - chore: Other changes that don't modify src or test files
 - revert: Reverts a previous commit

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3 Ignored Deployments

Name	Status	Preview	Comments	Updated (UTC)
ai	⬜️ Ignored (Inspect)	Visit Preview		Jun 15, 2024 2:53am
cal	⬜️ Ignored (Inspect)	Visit Preview		Jun 15, 2024 2:53am
calcom-web-canary	⬜️ Ignored (Inspect)	Visit Preview		Jun 15, 2024 2:53am

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