Allow enabling additional vulkan extensions in Adapter::open and Instance::init

[Vecvec]

Connections
Updated version of #7334
Fixes #7324 (including additional bit requested by @JMS55)

Description
Adds Adapter::open_with_callback & Instance::init_with_callback which have a callback that can add extensions to the device/instance respectively. Moves Adapter::open & Instance::init to be defined in terms of these functions so that they do not fall out of sync.

Testing
Like other hal functions it is untested.

Squash or Rebase?
Squash

Checklist

• Run cargo fmt.
• Run cargo clippy --tests.
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[JMS55]

I don't have time today, but before this is merged I would like to validate that it will work with https://github.com/JMS55/dlss_wgpu.