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

[flang] Introducing a method to dynamically and conditionally register dialect interfaces.  (#80881)

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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Co-authored-by: Vijay Kandiah <vkandiah@nvidia.com>