Allow querying all backends registered with the executorch runtime

[omerjerk]

Summary: Partially resolves: #8267

Differential Revision: D69754074

[pytorch-bot]

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This pull request was exported from Phabricator. Differential Revision: D69754074

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[swolchok]

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std::vector cannot be used in runtime/backend; it is part of ExecuTorch core. See https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#cc-standard-library-usage . (We should update that document to clarify what is in ExecuTorch core and what is not...)

[swolchok]

for std::vector. see top-level comment on PR. not sure offhand how to get this done without heap allocation but I imagine there are other examples of "enumeration functions" as mentioned in the issue. I would recommend just checking with issue owner @GregoryComer