refactor: move the embedding to the rag module and abstract the rerank runner for extension

[hwzhuhao]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

1.Add RerankRunnerFactory and BaseRerankRunner to simplify the creation logic of RerankRunner and enhance extensibility.
2.The core.embedding.cached_embedding depends on core.rag.datasource.entity.embedding, while core.rag.datasource.vdb relies on core.embedding.cached_embedding, creating a circular dependency.
3.Since the cached_embedding in the embedding package is only used by rag, move it to the rag module and reorganize it with the embedding_case module into a new package named embedding. This will make the structure clearer and the functionality more cohesive.
4.Rename embedding_constant to embedding_type. Since this module will be used by both model_runtime and rag, move it to core.entity.
After these changes, the embedding will mainly be used by rag, so moving it to the rag module will result in a clearer directory structure.
Scenario 1: Vector Search

Scenario 2: Hybrid Search

Fixes

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

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

• Test A
• Test B