Migrate to per-study file structure.

[goodov]

Update instructions, create per-study file structure, switch workflows to use the new generator.

This PR currently requires both seeds to be in sync. Once it's merged and the production CI is updated, the restriction will be lifted, and the old seed will no longer be used.

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1234.
   Android: Set the command line to --variations-pr=1234 in debug menu, restart the browser.
   iOS: Set Variations PR to 1234 in Brave Core Switches debug menu, restart the browser.
2. Wait 5-10 seconds to fetch the seed.
3. Restart the browser to apply the seed.
4. Ensure Active Variations section at brave://version starts with the expected seed version (see below).

Seed Details

Parameter	Value
Version	pull/1234@7dbfdcbdac5414ede435c4864407b899b6dc698c
Uploaded	Mon, 28 Oct 2024 06:52:30 GMT
PR commit	63b8479
Base commit	885cb07
Merge commit	7dbfdcb
Serial number	c5addc05f8e8386255c02b66ec784e2b

[github-actions]

[puLL-Merge] - brave/brave-variations@1234

Description

This PR significantly changes the structure of the Brave Variations repository. It removes the old seed-based system and introduces a new study-based system for managing feature rollouts and experiments. The changes include updates to workflows, documentation, and the addition of numerous study files.

Changes

Changes

1. .github/CODEOWNERS:

   • Updated ownership rules for the seed directory and studies.

2. .github/workflows/generate-test-seed.yml:

   • Removed Python setup and replaced seed generation with a new npm-based command.

3. .github/workflows/test-src.yml:

   • Added steps to compare Python and TypeScript seed generators.

4. README.md:

   • Updated repository structure description and instructions for working with studies.
   • Updated key generation and seed serialization processes.

5. src/README.md:

   • Added information about the new seed generator and tools.

6. src/scripts/lint.ts:

   • Enabled linting for seed tools.

7. New study files:

   • Added numerous .json5 files in the studies/ directory, each representing a different feature study or experiment.

Possible Issues

1. The transition from the old seed-based system to the new study-based system may require updates to dependent systems or processes.
2. The new seed generation process might behave differently from the old one, potentially affecting feature rollouts.

Security Hotspots

1. The changes to the seed generation and signing process (README.md) should be carefully reviewed to ensure no security vulnerabilities are introduced.
2. The new study files contain feature flags and experiment configurations. These should be audited to ensure they don't unintentionally expose sensitive features or configurations.