Command to execute query from Apollo CLI

It'd be helpful to have a command in the CLI to execute operations against a graphql server. The output could be piped to other tools

For example:
```
apollo execute "query(id: ID!) { node(id: $ID) { ... on User { username } } }" --variables=\{ "id": 1 \} --headers=authorization:"bearer 12345" | jq 'data.node.username' | cat
```

The logic and maintenance of this command would be pretty minimal if my thinking is correct.

Start by reviewing the Apollo CLI command structure and how existing commands handle arguments and output. Define the command's handling of GraphQL operations, variables, headers, and piped output, then verify it against the example and confirm the result can be consumed by tools such as jq.

Written by the indexing model from the issue text.