WIP: Input only sessions

[TheElixZammuto]

Description

This is a Draft PR for making Sunshine capable of input-only sessions.
Some clients, due to the platform limitations, are not able to have all the available input methods (for example the Xbox client does not allow mouse, touch and DS4 inputs due to limitations of the UWP API on Xbox)
With this PR, Sunshine can make sessions without the video and audio thread initialized, to make more lightweight sessions.

This is a first draft the server part of the change. While I have a working client and moonlight-common-c modifications, I would prefer to discuss this on the server side with you before going forward to the other parts of the stack.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[ivdimitro]

This will be ideal for VFIO use case when the passed through video can be attached to the same monitor but still send the mouse and keyboard so not to have to switch dongles.
Ideally it should be selectable to pass the sound as well.