feat: add support to load sub-graph from Tailcall

[tusharmath]

Depends on but not blocked on:

• refactor: add support for meta in @link #2821
• feat: expose config using an API #2817
• Feature Request: Support for Apollo Federation in Tailcall #2639
• Add support to set proxy in http and grpc directives #2820
• feat: reimplement merge_right using Apollo federation specification #2818
• feat: add reverse proxying capabilities on admin server #2819

Tailcall should be able to connect to other Tailcall subgraphs for and create a Federated GraphQL API.

Config Changes

schema 
  @link(src: "http://products:4000" type: SubGraph meta: {name: "Products"}) 
  @link(src: "http://products:5000" type: SubGraph meta: {name: "Reviews"}) {
  query: Query
}

Feature Requirements

• Tailcall should connect to the provided source and load the config.
• The loaded configuration should be merged automatically.
• The merged configuration should add http/grpc proxy while merging to /pass route.

Technical Requirements

• Write high-quality modular code.
• Ensure enough tests are written to guarantee everything is working.
• Merging should fail if the sub-graph isn't Tailcall.

[tusharmath]

/bounty $300

[algora-pbc]

## 💎 $300 bounty • Tailcall Inc.

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