feat(mat-form-field): Support nested content projection for _formFieldControl

[bamblack]

Feature Description

Allow the MatFormFieldControl within the mat-form-field component to be accessed through nested content projection

Use Case

I have a use case where I'm attempting to write a component that wraps mat-form-field for the purpose of "easier" error configuration, allowing devs to pass a { key: string, message: string }[] to my component. I have a simple StackBlitz showing what currently "works" and how I'd like it to work: https://stackblitz.com/edit/components-issue-starter-yqzq6dje?file=src%2Fcustom-form-field-wrap.ts. Basically, I think if you were to add { descendants: true } to the definition of _formFieldControl in mat-form-field it would allow my use case. Although, I'm not sure what other, if any, repercussions of that change would be.

[bamblack]

Well, after doing research it looks like my suggested action wouldn't fix it, because the default of descendants is already true and ContentChild can't pierce multiple levels of content projection through components like I thought it could. Back to the drawing board.