[flang] Introducing a method to dynamically and conditionally register dialect interfaces.

[VijayKandiah]

This change introduces the addFIRExtensions method to dynamically and conditionally register dialect interfaces. As a use case of addFIRExtensions, this change moves the static registration of FIRInlinerInterface out of the constructor of FIROpsDialect to be dynamically registered while loading the necessary MLIR dialects required by Flang. This registration of FIRInlinerInterface is also guarded by a boolean addFIRInlinerInterface which defaults to true.

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