[mlir][python] Allow contexts to be created with a custom thread pool. (#72042)

The existing initialization sequence always enables multi-threading at
MLIRContext construction time, making it impractical to provide a
customized thread pool.

Here, this is changed to always create the context with threading
disabled, process all site-specific init hooks (which can set thread
pools) and ultimately enable multi-threading unless if site-configured
to not do so.

This should preserve the existing user-visible initialization behavior
while also letting downstreams ensure that contexts are always created
with a shared thread pool. This was tested with IREE, which has such a
concept. Using site-specific thread tuning produced up to 2x single
compilation job behavior and customization of batch compilation (i.e. as
part of a build system) to utilize half the memory and run the entire
test suite ~2x faster. Given this, I believe that the additional
configurability can well pay for itself for implementations that use it.
We may also want to present user-level Python APIs for controlling
threading configuration in the future.