Mark IInboundSignalMessage as alpha and readonly

[CraigMacomber]

Description

IInboundSignalMessage is the last public API in @fluidframework/runtime-definitions, and is only exposed via alpha APi, like the signal event on the alpha IFluidDataStoreRuntime.

Breaking Changes

Users of IInboundSignalMessage will need to import it from the /legacy scope and should not mutate it. Only users of existing @alpha APIs like IFluidDataStoreRuntime should be able to use this type, so it should not introduce new /legacy usage.

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