[relay-compiler] not yet implemented: InputObjectTypeExtension

## Problem

When using an `extend input` syntax, the relay compiler panicked and failed to generate. An example schema would be:

```graphql
input HelloInput {
name: String
}

extend input HelloInput {
age: Int
}

type Query {
hello(input: HelloInput): String!
}
```

## Expected

The relay compiler should be able to handle an `extend input` syntax that is in the [GraphQL spec (3.3.2. Schema Extension)](https://spec.graphql.org/October2021/#sec-Schema-Extension).

## Reproduce

https://github.com/GalvinGao/relay-compiler-not-implemented-reproduce

Clone and `yarn install`. Then, run `yarn build:relay`.

```
❯ yarn build:relay
yarn run v1.22.19
$ relay-compiler
[INFO] querying files to compile...
[INFO] [default] compiling...
thread 'main' panicked at 'not yet implemented: InputObjectTypeExtension', crates/schema/src/in_memory/mod.rs:770:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error Command failed with exit code 101.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
```