[mlir][bufferization] Drop equivalent buffer results - modify public functions option (#177589)

Adds an option to allow modification of public functions to the drop
equivalent buffer results API and the respective pass.
A new standalone test for the pass is also added.

https://github.com/llvm/llvm-project/pull/163001 modified the default
behavior by disallowing rewriting public function. This PR preserves the
new behavior and only adds an opt-in flag to reenable changing public
function.

The extra flag aligns dropping equivalent function results with another
bufferization API `promoteBufferResultsToOutParams` which also allows
opt-in public function rewriting.

GitOrigin-RevId: 9a767dce95aeba9ad10253d86a67d7e504db5d58