Support for struct first schema, using go tags

### What happened?
Because go already have structs, that can define schema, a support for code first approach is potentially possible.
### What did you expect?
i expect the schema to be able to made using tags on the structs, which will help on having your schemas on one place, which will make working easier.
Plus it is possible to use both approaches.

### versions
all version

The issue proposes adding struct-tag-based schema generation to gqlgen, a Go GraphQL server library. Start by examining the existing code-first and schema-first approaches in the codebase, likely in the `codegen` and `graphql` packages. Look for how schemas are currently parsed and generated. 'Done' would be a proof-of-concept where a Go struct with specific tags can produce a valid GraphQL schema.

Written by the indexing model from the issue text.