Implement abstraction for marshalling direction in the generator APIs

[jkoritzinsky]

Use MarshalDirection to provide a nice abstraction for determining whether we're marshalling a parameter/return value/etc from managed to unmanaged or vice versa. This abstraction will be useful when enabling unmanaged->managed stubs as we won't need to go update every marshalling generator to correctly understand what to do.

Also rename some members from "in/out/ref" to use the direction-based names.

Extracted from #77130

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Issue Details

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Use MarshalDirection to provide a nice abstraction for determining whether we're marshalling a parameter/return value/etc from managed to unmanaged or vice versa. This abstraction will be useful when enabling unmanaged->managed stubs as we won't need to go update every marshalling generator to correctly understand what to do.

Also rename some members from "in/out/ref" to use the direction-based names.

Extracted from #77130

Author:	jkoritzinsky
Assignees:	-
Labels:	area-System.Runtime.InteropServices, source-generator
Milestone:	8.0.0

[jkoritzinsky]

All failures are tracked by known issues (#78599), so merging in.