feat(gql): download schema

[gorillamoe]

Add a command to download a graphql schema.

The schema file is named after the buffer name, without the .http extension with the suffix .graphql-schema.json.

So if the name of the current buffer is test.http, the downloaded schema file would be named test.graphql-schema.json.

Once such a schema file has been downloaded, you can have autocompletions via kulala-cmp-graphql.nvim.

TODO: make the introspection query a dedicated file.

Closes #448