Redesign MimeType

[whyoleg]

Main changes:

• rename to RSocketMimeType
• move to root package
• use nested classes

POC API:

public sealed interface RSocketMimeType {
    public sealed interface WithName : RSocketMimeType {
        public val name: String
    }

    public sealed interface WithIdentifier : RSocketMimeType {
        public val identifier: Byte
    }

    public sealed interface WellKnown : WithIdentifier, WithName

    public companion object {
        public val ApplicationAvro: WellKnown
        public val ApplicationCbor: WellKnown
    }
}

public fun RSocketMimeType(name: String): RSocketMimeType.WithName
public fun RSocketMimeType(identifier: Byte): RSocketMimeType.WithIdentifier

Similar API design should be done to auth and metadata types