Adds ability to create files/folders in subfolder using DynamicForm. Closes #1901

[martinlingstuyl]

Closes #1901

Q	A
Bug fix?	[ ]
New feature?	[x]
New sample?	[ ]
Related issues?	Closes #1901

What's in this Pull Request?

In its current state, a dynamic form always creates new items/files/folders in the root folder of a list or library. This is not very handy when using the dynamic form on document libraries. For example as a custom form to create document sets.

This PR adds the ability to create files, folders and document sets in subfolders.

I've added an extra option to do this. So the user has full control over the folder that should be used. In this way it doesn't matter how the form is rendered. A RootFolder querystring parameter may be available that the user can use. (For example in the NewForm of an SPFx FormCustomizer) Or it may not be available. The user is in control of what value he supplies...

The feature does not work for list items. Even if list items can also be created in folders. I've not added that currently as it was a little bit more work that I'm not sure is very relevant.

Testing instructions

To test this feature, you'll need a library where you can create document sets. In TestForm.tsx fill in the contentTypeId, folderPath AND hiddenFields properties as follows:

<DynamicForm
          context={this.props.context}
          contentTypeId='0x0120D52000796BC7FCC8B819488729FB2B77B0E799002B2379630DCCB04D883603B763E843D7'
          listId={this.props.context.list.guid.toString()}
          listItemId={this.props.context.itemId}
          hiddenFields={['FileLeafRef']}
          folderPath='Some subfolder/Some deeper subfolder'
          onListItemLoaded={async (listItemData: any) => { // eslint-disable-line @typescript-eslint/no-explicit-any
            console.log(listItemData);
          }} />

For the folderPath option, you can also use a server relative URL. For example when using the RootFolder querystring of an SPFx form customizer. However: it should be a path to the same library, otherwise the code will fail.

FileLeafRef should be excluded in hiddenFields, otherwise the form will not submit. This is supposed to be fixed in #1906.

[martinlingstuyl]

@joelfmrodrigues, there it is. Hope its up to the repo standards 😉 We didn't discuss a lot beforehand, so I just took a course that seemed most logical to me. Do let me know if you've got a different opinion!

[joelfmrodrigues]

@martinlingstuyl could you please resolve the conflicts?

[martinlingstuyl]

@joelfmrodrigues, done!

[joelfmrodrigues]

@martinlingstuyl many thanks, this is now merged