…#16103)

## Summary

This pr would allow us to import a generic from the plugin and then use
it. The limitation with json-schema-to-typescript is the inability to
generate a type in the file that takes generics.

There are some generated generics plopped in, internally by our
addSelectGenericsToGeneratedTypes function.

This PR exposes a way for external plugins to manipulate the generated
config post-processing of json-schema-to-typescript which is not
possible today.

## Implementation

In `packages/payload/src/bin/generateTypes.ts`, after
`addSelectGenericsToGeneratedTypes`:

```ts
if (config.typescript?.postProcess?.length) {
  for (const fn of config.typescript.postProcess) {
    compiled = fn(compiled);
  }
}
```

Add to TypeScript config type:
```ts
typescript?: {
  postProcess?: Array<(compiled: string) => string>;
}
```

**Use cases**

- Inject imports from plugin packages
- Add utility types

**Example**

```ts
// Plugin adds an import and uses it via tsType on fields

typescript: {
  postProcess: [
    ...(config.typescript?.postProcess || []),
    (compiled) => {
      const importStmt = `import type { MyPluginType } from '@payloadcms/my-plugin';`;
      return compiled.replace(/(\/\*[\s\S]*?\*\/\n)/, `$1\n${importStmt}\n`);
    },
  ],
}
```

**Notes**

- Follows existing pattern (Payload already post-processes for Select
generics)