Add basic working set support

[cmrigney]

Working Sets

Working sets allow you to organize and manage collections of MCP servers as a single unit.

See docs/working-sets.md for more info and limitations of the current implementation.

Tests will be coming in a follow up PR.

Quickstart

# Enable the feature
docker mcp feature enable working-sets

# Create a working set
docker mcp workingset create --name dev-tools \
  --server docker://mcp/fetch

# Use it with the gateway
docker mcp gateway run --working-set dev-tools