executorch [executorch] Migrate runtime/executor to new namespace

Stack from ghstack (oldest at bottom):

#4621
#4620
#4619
#4618
#4617
#4616
-> #4615
#4614
#4613
#4612
#4611
#4610
#4609
#4608
#4607
#4606
#4605

Migrate these headers to the new ::executorch::runtime namespace. Add temporary aliases from the old ::torch::executor namespace so we can migrate users incrementally.

The tests for these headers are updated in the next diff to help demonstrate that the backwards-compatibility aliases work.

Differential Revision: D60703365

Aug 09 '24 00:08 dbort

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/4615

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