Allow IBMServerlessProvider users to save their credentials to disk

[caleb-johnson]

Fixes #1042

[IceKhan13]

Should we use what QiskitRuntimeService already have? 🤔

https://github.com/Qiskit/qiskit-ibm-runtime/blob/dc3b1d0d4a8f9426726e5dd9cc74d6373fd68709/qiskit_ibm_runtime/qiskit_runtime_service.py#L683

then implementation would be something like this:

class IBMServerlessProvider(...):
    def __init__(self, token: Optional[str] = None, name: Optional[str] = None):
         if token is None:
              self.token = QiskitRuntimeService(name=name).token
         else:
              self.token = token

    @classmethod
    def save_account(cls, token: Union[str, QiskitRuntimeService], overwrite: bool = True):
           if isinstance(token, QiskitRuntimeService):
               QiskitRuntimeService.save_acoount(**token.get_account())
           else:
               # token string
               QiskitRuntimeService.save_account(token=token, channel=..., ...)

In that case it will be same file as runtime is using.

But in general I'm ok with any option :)

[caleb-johnson]

Should we use what QiskitRuntimeService already have? 🤔

https://github.com/Qiskit/qiskit-ibm-runtime/blob/dc3b1d0d4a8f9426726e5dd9cc74d6373fd68709/qiskit_ibm_runtime/qiskit_runtime_service.py#L683

then implementation would be something like this:

class IBMServerlessProvider(...):
    def __init__(self, token: Optional[str] = None, name: Optional[str] = None):
         if token is None:
              self.token = QiskitRuntimeService(name=name).token
         else:
              self.token = token

    @classmethod
    def save_account(cls, token: Union[str, QiskitRuntimeService], overwrite: bool = True):
           if isinstance(token, QiskitRuntimeService):
               QiskitRuntimeService.save_acoount(**token.get_account())
           else:
               # token string
               QiskitRuntimeService.save_account(token=token, channel=..., ...)

In that case it will be same file as runtime is using.

But in general I'm ok with any option :)

Yes, I think this is a better solution. Let me take a look..

[IceKhan13]

Looks clean! :)

Feel free to merge after docstring update