DOI: make default option selection configurable

[ntarocco]

The UI widget at the moment shows by default the text input where you insert your already pre-existing DOI.
We should change this and make it configurable, as in CDS most of the users will not have a DOI.

[sakshamarora1]

Summary of the discussions for optional DOI UI

Backend:

1. The functionality already exists, can be configured by adding the following lines to the invenio.cfg file:

from invenio_rdm_records.config import RDM_PERSISTENT_IDENTIFIERS, RDM_PARENT_PERSISTENT_IDENTIFIERS
RDM_PERSISTENT_IDENTIFIERS["doi"]["required"] = False
RDM_PARENT_PERSISTENT_IDENTIFIERS["doi"]["required"] = False

2. Needs a bugfix on this line when required is false, it errors

Frontend:

1. To implement the following UI when required is set to false.
   
2. The required=true config flag will show 2 radio button, while required=false will show the 3rd one
3. When the user selects No, I need one, the user must click on Get a DOI now button , otherwise the validation should fail. (Draft save or publish)
4. Make the selection of the default radio button configurable. (explore the possibility to have a 4th option for none default that will force the user to select one of the options)

Discussion link: inveniosoftware/product-rdm#205