[Content] Add generateId option for file loader

[marceloverdijk]

Body

Summary

Add a generateId (or idAccessor) option for the file loader, similar as the glob loader already has.
This to handle JSON files which do not have id field.

Background & Motivation

Some JSON files do not have a id field, or a alternative field that is unique.

These files cannot be used as content collections without altering them, which is not always possible (think about externally provided JSON files).

A generateId or idAccessor option (similar as the glob loader already has) could provide this flexibility:

export interface GlobOptions {
	/** The glob pattern to match files, relative to the base directory */
	pattern: string;
	/** The base directory to resolve the glob pattern from. Relative to the root directory, or an absolute file URL. Defaults to `.` */
	base?: string | URL;
	/**
	 * Function that generates an ID for an entry. Default implementation generates a slug from the entry path.
	 * @returns The ID of the entry. Must be unique per collection.
	 **/
	generateId?: (options: GenerateIdOptions) => string;
}

Goals

A concise, bulleted-list outlining the intended goals of this RFC.

• Support JSON files for content collections that don't have a id field (but e.g. a year field that is the unique key).
• Support JSON files for content collections that don't have single field that is unique.

Example

Using a idAccessor?: string | ((data: any) => string); the file loader could look like:

Simply expects a id field (as-is):

const persons = defineCollection({
  loader: file('data/persons.json'),
  schema: z.object({
    ..
  }),
});

Use a field other than id representing the unique key:

const seasons = defineCollection({
  loader: file('data/seasons.json', {
    idAccessor: 'year',
  }),
  schema: z.object({
    ..
  }),
});

Generate a customized id based on multiple fields:

const monthlyRevenue = defineCollection({
  loader: file('data/monthly-revenue.json', {
    idAccessor: (data) => `${data.year}::${data.month}`,
  }),
  schema: z.object({
    ..
  }),
});

[marceloverdijk]

I've created a PR (withastro/astro#12308) which adds a generateId option, similar as the glob loader.
I choose the generateId option over the idAccessor as outlined above for consistency between loaders.

I hope the idea is clear, and that you will see the benefit of this proposed change.