Conditional embedding of field types

[darrenmothersele]

There is an example of the expected functionality, but using Formly directly, in this "repeat" example https://stackblitz.com/edit/angular-rxlpp4-qtkqng?file=src%2Fapp%2Frepeat.component.ts

Something similar is required for JSONSchema generated forms where sub-schema of child elements could be selected from a list of optional item types.

[dahacouk]

Merge with #59 and #79