Embedded OpenXR Layer Support

[rblenkinsopp]

Summary

This MR:

• Adds a new OpenXR Feature which installed our API layer as an embedded layer
• Dynamically removes LeapC and The Android Service Binder from these builds, so they don't conflict
• Adds a new feature group to group them together.
• Adds a warning if the OpenXR version in Unity is less that 1.0.25 as the best proxy for the loader version in use which can't be checked directly.

It is currently a draft as it doesn't include the final version of the API layer that will ship with this option, but is ready for review.

Contributor Tasks

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• Pair review with a member of the QA team.
• Add any release testing considerations to the MR for the next release.
• Check any relevant CHANGELOG files have been updated.
• Ensure documentation requirements are met e.g., public API is commented.
• Consider any licensing/other legal implications for this MR e.g. notices required by any new libraries.
• Add any relevant labels such as breaking to this MR.
• If this MR closes a Jira issue, make sure the fix version on the JIRA issue is set to the correct one.

Reviewer Tasks

Add any instructions or tasks for the reviewer such as specific test considerations before this can be merged.

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• Code reviewed.
• Non-code assets e.g. Unity assets/scenes reviewed.
• Documentation has been reviewed. Includes checking documentation requirements are met and not missing e.g., public API is commented.
• Checked and agree with release testing considerations added to MR for the next release.

Closes JIRA Issue

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[rblenkinsopp]

I've resolved the non-naming-based issues with this.

[rblenkinsopp]

We agreed to hold off on merging this till Unity has a default loader which supports embedded layers (>= 1.0.25); this will remain in draft until then.

[rblenkinsopp]

This PR also fixes the namespacing of the asmdefs to match the Leap.Tracking.OpenXR namespace for core OpenXR support and the embedded API layer.