make models `RecordModel|AdminModel|null` use a base type `BaseModel` to avoid use of `dynamic`

[Luckey-Elijah]

There are 6 occurrences of the "union" RecordModel|AdminModel in the pockebase/dart-sdk:

• https://github.com/pocketbase/dart-sdk/blob/master/README.md?plain=1#L187
• https://github.com/pocketbase/dart-sdk/blob/master/lib/src/async_auth_store.dart#L58
• https://github.com/pocketbase/dart-sdk/blob/master/lib/src/auth_store.dart#L9
• https://github.com/pocketbase/dart-sdk/blob/master/lib/src/auth_store.dart#L19
• https://github.com/pocketbase/dart-sdk/blob/master/lib/src/auth_store.dart#L26

Since dart does not have true unions I propose we implement some abstract parent type or with some mixin to provide "better than dynamic" types for locations in order to provide a better developer experience.

[ganigeorgiev]

With the planned refactoring this will be normalized and everything will be a Record.