mcp: proxy stdio servers in standalone mode

[nacx]

Description

Opening as a draft to get early feedback.

When running in standalone mode, it allows running and proxying stdio MCP servers. This PR only supports defining the servers in the servers.json format, as for now, we don't want to introduce new nouns for a feature that does not apply to Kubernetes.

With this change, the CLI can be started with a file like the following:

aigw run --mcp-json '{
  "mcpServers": {
    "everything": {
      "command": "npx",
      "args": ["-y","@modelcontextprotocol/server-everything"]
    },
    "filesystem": {
      "command": "npx",
      "args": ["-y","@modelcontextprotocol/server-filesystem"]
    }
  }
}'

Related Issues/PRs (if applicable)

Depends on envoyproxy/gateway#7427 for a clean implementation.

Special notes for reviewers (if applicable)

N/A

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.95%. Comparing base (2fa6b27) to head (9d3f3e8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1479      +/-   ##
==========================================
- Coverage   83.96%   83.95%   -0.01%     
==========================================
  Files         143      143              
  Lines       12618    12618              
==========================================
- Hits        10595    10594       -1     
- Misses       1406     1408       +2     
+ Partials      617      616       -1     

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