feat: add multi knowledgebase

[Abirdcfly]

What type of PR is this?

/kind cleanup
/kind feature

What this PR does / why we need it

has tested in gpu 167 env, old application also works.
can update to new app node by update app config in web ui.

The above gif shows a complex application that uses 2 knowledgebases, one knowledgebase to provide knowledge of attendance and one knowledgebase to provide knowledge of arbiter and the application uses multiquery and rerank.

Which issue(s) this PR fixes

Fixes #

Special notes for your reviewer

1. add mergerRetriever to merge multi-knowledgebase like langchain python does.
2. add conversation-knowledgebase-placeholder to appruntime and app nodes, this placeholder clearly indicates that there may be a conversation knowledgebase here.
3. add knowledgebases to updateApplicationConfig gql input and resp for support multiquery knowledgebases, also compatible with the old single knowledgebase model. ( @Carrotzpc front end also need update to support multiquery knowledgebases.)
4. add tests.