Implement adapter instance handles

[callumfare]

Corresponding changes to the adapters, pi2ur, etc are in intel/llvm#10349

Summary of new/changed entry points:

• urInit Adapters no longer perform global state setup in urInit. The loader may still perform setup (layer config etc), so the entry point is still needed, but it might be possible to combine it with the below.
• urAdapterGet Individual adapters return exactly 1 adapter handle, the loader returns however many adapters are known. Each returned adapter increments the reference count of the adapter by 1. When the reference count goes from 0 to 1, adapters may perform setup of global state (which previously happened in urInit). The underlying object pointed to by the ur_adapter_handle_t should be a single static object in the adapter.
• urPlatformGet Now explicitly takes the adapters to use as arguments. Multiple adapters can be used in the same call. This allows all platforms available to be queried as before, or more fine selection of platforms from certain adapters.
• urAdapterRetain/Release Increment or decrement the adapter reference count. When the reference count drops to 0, adapters may perform teardown of global state (which previously happened in urTearDown).
• urTearDown Adapters no longer perform global state teardown in urTearDown. This entry point may be kept if useful for the loader, otherwise it could be removed.
• urAdapterGetLastError Previously urPlatformGetLastError. No changes other than it takes an adapter handle rather than platform handle, removing the need to always have a platform handle when a UR entry point is called to be able to process an adapter-specific error.
• urAdapterGetInfo Can be used to query the backend of the adapter (which is just a duplicate of UR_PLATFORM_BACKEND).

Addresses #714 by allowing better sharing of UR adapters when multiple modules in an application share the UR libraries. They may independently call urAdapterGet safely, or may explicitly pass adapter handles around (retaining/releasing as necessary).

Provides some more programmatic control over which adapters are used by allowing users to query which backend is supported by each adapter, and only fetching platforms from certain adapters.

Fixes intel/llvm#10066 by allowing piPluginGetLastError to be implementable in pi2ur.

[callumfare]

I've updated the tests but there are still CI failures. I'll look into those, but the actual spec and loader changes should be ok so feedback is still welcome

[kbenzie]

Feels odd to be passing in all the adapters here as a list, its a different approach to urDeviceGet only taking a single platform. I can see why this is a less intrusive way to do it though.

[callumfare]

Yeah, I did it this way to keep the changes as unobtrusive as possible, especially since urPlatformGet is in pretty much every example and test.

It also means fewer changes in the loader, which already had a special case for urPlatformGet where it looped over every adapter in the loader context. This PR just changes that to the adapters passed in.

If consistency with the other entry points is important I could look into changing this.

[kbenzie]

I don't think its a blocker for this work but perhaps something to look into at a later point.

[callumfare]

Made a note to revisit this in #728

[pbalcer]

In a separate PR we might also want to update urInit and urTearDown documentation.

[veselypeta]

LGTM