Fix update image block type

[cfeeney5]

Summary

This update adds the newly available slack_file object to the imageBlock and sets the image_url to be optional

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• [X ] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
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[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @cfeeney5 to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Thank you for taking the time to send this! As I mentioned at #1734 (comment), a similar change is required on the image block element side. Also, we prefer union types for this.

[filmaj]

Thanks for sending a PR for this! Much appreciated.

However, we will require a few changes:

1. We should factor the Slack File object into its own type, and, because it is documented as a 'composition object' on our API site, it should belong in the same area of the types project: composition-objects.ts. We also want to make sure to give it a top-level JSDoc, with links to the API - just like the other objects in this source file. If you could please follow that style, that would be great!
2. There is a subtle detail in the Slack File composition object that is described in the docs:

This file must be an image and you must provide either the URL or ID. In addition, the user posting these blocks must have access to this file. If both are provided then the payload will be rejected.

So either url or id must be provided - but not both. Similarly, either image_url or slack_file must be provided - but not both. We should express this in TypeScript, using the union type, as @seratch mentioned.

So both the slack_file object should itself be a union type, and the ImageBlock (and ImageElement that once more Kaz pointed out also needs to be updated in the same way), need to express this either-or constraint.

Something like (with, of course, nice JSDocs and links to our API site, etc.):

interface SlackFileViaURL = {
  url: string;
}
interface SlackFileViaID = {
  id: string;
}
type SlackFile = SlackFileViaURL | SlackFileViaID;
interface ImageElementWithImageURL = {
  image_url: string;
}
interface ImageElementWithSlackFile = {
  slack_file: SlackFile;
}
type ImageElement = {
  type: 'image';
} & (ImageElementWithImageURL | ImageElementWithSlackFile);

The benefit to this approach is that the required properties are now properly expressed as required properties and we are able to fully model the exclusive-or constraint on these types.

[cfeeney5]

Ive refactored this now for use with unions. UrlImageObject and SlackFileImageObject are exported from block-elements so it can be used there and in blocks. Im not sure which way is preferred.

[filmaj]

Nice, getting closer! Left some more suggestions and a few (hopefully final) change requests.

[filmaj]

I've enabled the CI tests for this PR and it seems the linter is now failing. Could you please also fix those up? I think many of the formatting changes you introduced will not be allowed according to our linter configuration / style guide.

[filmaj]

#1783 has superseded this PR; closing.

[filmaj]

FYI the fix for this has been released to npm as @slack/types v2.12.0.