Make credentials purpose required

[thclark]

Feature request

Use Case

In the credentials strand, the purpose field is not required, just the credential name. We discussed in this PR that it would be sensible to make purpose required.

It should be a unicode character field of up to 120 characters. I suppose the actual character string could be blank, but have the field always present.

Current state

In the examples we're providing a purpose field all the time anyway, and think it's best practice to properly describe the entry so that should be reinforced to support good UX