Allow to skip field rendering still allowing its server-side processing

A custom admin-field I developped is dynamically creating subfields on the fly.

For example:
```yaml
header.form.field_selection:
type: my-custom-type
fields:
firstname:
content: First name
```
would submit `data[header][form][firstname]` **and** `data[header][form][_is_required][firstName] = 0 or 1`

Since `_is_required` is not `known` to the blueprint, it ends-up in `blueprint.extra`, thus creating a hidden element under `[data][_json]` which, sadly, *always* take-over my submitted values
=> data is not saved.

In order to avoid that, I added a:
```yaml
header.form._is_required:
type: hidden
```
to my form.

The problem is that with such a hidden field, the POST submission looks like:
```
data[header][form][_required] = ""
data[header][form][_required][firstname] = 1
```
=> data is not saved because the array structure is disregarded.

No file or test is named. Start by tracing the custom admin-field and hidden-field submission behavior described in the issue, including blueprint.extra and the server-side POST processing. Done means dynamically created subfields can be processed without a rendered hidden field overriding the submitted array values.

Written by the indexing model from the issue text.