Make `enabled_extensions` public in Vulkan backend

[LaylBongers]

This is required for OpenXR support, when creating a custom device gfx-hal needs to be queried for what raw vulkan extensions to enable for the given gfx-hal features.

PR checklist:

• [x] make succeeds (on *nix)
• [x] make reftests succeeds
• [x] tested examples with the following backends:
  • gfx_backend_vulkan

[kvark]

No problem exposing this, but seems rather suspicious :) We'll see how you are using it. bors r+

[bors[bot]]

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