provide a plugin mechanism for core/rpc/storage

[darkskygit]

currently we have the following features coupled in core/rpc/storage:

• core: full text indexing, single page extract
• rpc: websocket/webrtc connector
• storage: image minify, stream storage, database/s3 support

these features need to introduce additional dependencies, making testing and compiling time longer

at present, cargo features are used to switch these functions, which requires us to include a lot of conditional judgments in the code, and making the code difficult to maintain

therefore, we need to design a plugin mechanism to make these features pluggable, which makes the code easier to maintain and facilitates external developers easily to develop new functions