## Description

Create a `impl_string_input` macro that opts a type that implements
`FromStr` into being treated as a `ScalarType`, while still appearing as
a `String`.

This allows `async-graphql` to handle input validation as part of its
own query resolution, which avoids us repeating validation work
through-out the codebase.

The `Digest` type is the first type to use the macro (although the
pattern is already used by the `Cursor` type), in this PR.

## Test Plan

```
sui-graphql-rpc$ cargo nextest run
```

## Stack
- #15467 
- #15470 
- #15471 
- #15472 
- #15473
- #15474 
- #15475 
- #15484 
- #15485
- #15519
- #15520
- #15521
- #15522
- #15523
- #15524
- #15525
- #15526