Hey guys, as discussed I've changed the name to a more general one
`PvfExecKind`, is this good or too general?
Creating this as a draft, I still have to fix the tests.

Closes #1585

Kusama address: FkB6QEo8VnV3oifugNj5NeVG3Mvq1zFbrUu4P5YwRoe5mQN

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Co-authored-by: command-bot <>
Co-authored-by: Marcin S <[email protected]>

Co-authored-by: Marcin S <[email protected]>

- Async-backing related primitives are stable `primitives::v6`
- Async-backing API is now part of `api_version(7)`
- It's enabled on Rococo and Westend runtimes

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Signed-off-by: Andrei Sandu <[email protected]>
Co-authored-by: Andrei Sandu <[email protected]>