[Investigation] Add MCP support to remote execution flows

Problem

Locally executed Agent Platform flows and Agentic Chat have access to MCP (see https://gitlab.com/gitlab-org/gitlab/-/issues/519938) but remotely executed ones since they are currently based on the Go executor.

As soon as available a headless version of the Node executor CLI will replace the Go executor but even then we do not have a way to provide an MCP configuration.

Desired Outcome

Add MCP support for remotely executed workflows.

Phase 1:

1. Local MCP servers, that don't require auth (e.g. Playwright, local Knowledge Graph MCP)
2. MCP server are configured / injected by GitLab into the remote execution environment, not configurable by end users right now.
3. Which MCP servers should be used should be set up in the flow config.

Phase 2:

1. Remote MCP servers
2. End-user configuration of MCP servers