Refactor `IceServer`'s password typing

[evdokimovs]

Synopsis

We need to add the IcePassword type for more convenient ICE password typing.

Checklist

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[evdokimovs]

FCM

Refactor `IceServer`'s password type (#190)

- add `IcePassword` type to the Client API
- change `IceServer::credential` type to `IcePassword`