Allow the creation of PDAs without fields

[julianzamt]

Is your enhancement request related to a problem? Please describe.
Since PDAs are used not only to store data, but to allow programs to sign txs, it may be useful to allow to define a PDA without fields.
Usually, these PDAs are not even created, but only used to sign cpi calls.

Depending on the use case, this PDA should be checked or not.

Now it is not possible to define in the CIDL a type without fields, thus forcing the need of a workaround to use this pattern.

Describe the solution you'd like
Allow the creation of at least PDAs without fields.
Not sure if this is needed also for Not PDAs.

Describe alternatives you've considered
Workaround: send it as an accountInfo, but proper checks must be added manually.