Put storage behind an interface in `irmaclient`

[w-ensink]

I propose an architecture that looks somewhat like this:

classDiagram
    class IdemixCredentialStorage {
        <<interface>>
        +StoreIdemix(credential)
        +LoadIdemix(id)
    }

    class LogStorage {
        <<interface>>
        +StoreLog(log)
        +LoadLogs()
    }

    class SdJwtVcStorage {
        <<interface>>
        +StoreSdJwtVc(credential)
        +LoadSdJwtVc(id)
    }

    class Storage {
        +StoreSdJwtVc(credential)
        +LoadSdJwtVc(id)
        +StoreIdemix(credential)
        +LoadIdemix(id)
        +StoreLog(log)
        +LoadLogs()
    }

    IdemixCredentialStorage <|-- Storage
    SdJwtVcStorage <|-- Storage
    LogStorage <|-- Storage

    class IrmaClient
    class OpenID4VPClient
    
    IrmaClient *-- IdemixCredentialStorage
    IrmaClient *-- LogStorage
    OpenID4VPClient *-- SdJwtVcStorage
    OpenID4VPClient *-- LogStorage

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This will keep components nicely separated.