Allow to set `mcp_storage_key` based on MCP server configuration file

[yankovs]

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Is your feature request related to a problem? Please describe.

Right now, when a new MCP server is entered through the UI, it is persisted using local storage using a key called mcps_storage_key. This makes chainlit request /mcp for each server stored in local storage

It makes it really hard to use pre configured servers, the app.py file has no easy way to control the local storage of the browser so hacky code needs to be written to emulate the calls to /mcp

Describe the solution you'd like

Allow to use a mcps.json file or something similar with a list of exisitng MCP server. Make mcps_storage_key be loaded with those MCPs from the config

Describe alternatives you've considered

A way to emulate this right now is to connect_mcp() each server in the decorators on_chat_start and on_chat_resume. This is like calls to /mcp but it's pretty hacky, a better way should be provided

Related: https://github.com/Chainlit/chainlit/issues/2148