[Form] Add "reindex" option for the CollectionType

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12936 
| License       | MIT
| Doc PR        | TODO

Adds a new option to the collection field type. This option allows to reindex the data array if necessary.

TODO:
- [ ] submit changes to the documentation