[Enhancement] be able to override MLX_METAL_VERSION when running cmake

[davidkoski]

Describe the bug

mlx-swift needs to have the various preambles (e.g. jit directory with metal shaders) built and checked in to the repository. Currently cmake picks up the SDK that is present on the machine doing this work but the actual jit happens using the runtime OS (or perhaps the minimum OS target). Anyway, I need to override MLX_METAL_VERSION to METAL_3_0 for generating these files (longer discussion offline about what that means and where we might go with it).

Ideally I would be able to override the MLX_METAL_VERSION e.g. with cmake .. -DMLX_METAL_VERSION=METAL_3_0

To Reproduce

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Expected behavior
Ideally I would be able to override the MLX_METAL_VERSION e.g. with cmake .. -DMLX_METAL_VERSION=METAL_3_0

Desktop (please complete the following information):

N/A