Feature request: `fileAssetJobSettings` for file-asset publishing jobs

`GitHubWorkflowProps` exposes `dockerAssetJobSettings` to inject `setupSteps` and extra `permissions` into Docker-asset publishing jobs, but there is no equivalent for **file-asset** publishing jobs (`Assets-FileAsset*`).

The [`npm install --no-save cdk-assets` step in `jobForAssetPublish()`](https://github.com/cdklabs/cdk-pipelines-github/blob/7c1ee2d26cdb173310c6d387e90f6775eedf1bba/src/pipeline.ts#L602-L605) is hardcoded, so there is no way to configure the npm registry, inject credentials, or add any pre-install step through the public API.

### Proposed API

```ts
export interface FileAssetJobSettings {
/** Steps to execute before installing cdk-assets and publishing. @default [] */
readonly setupSteps?: github.JobStep[];
/** Additional permissions for file-asset publishing jobs. @default - none */
readonly permissions?: github.JobPermissions;
}

export interface GitHubWorkflowProps extends PipelineBaseProps {
/** Job level settings applied to all file asset publishing jobs. @default - none */
readonly fileAssetJobSettings?: FileAssetJobSettings;
}
```

`setupSteps` would be injected immediately before the `Install` step in `jobForAssetPublish()`, consistent with how `dockerAssetJobSettings.setupSteps` is handled for Docker asset jobs.

### Use case

Any organisation that needs to configure npm before install - private registry, npm proxy, authentication - currently has no way to do so for file-asset jobs.

### Would you be willing to submit a PR?

Yes.