XCM paid execution barrier supports more origin altering instructions (#5917)

The AllowTopLevelPaidExecutionFrom allows ClearOrigin instructions
before the expected BuyExecution instruction, it also allows messages
without any origin altering instructions.

This commit enhances the barrier to also support messages that use
AliasOrigin, or DescendOrigin. This is sometimes desired in asset
transfer XCM programs that need to run the inbound assets instructions
using the origin chain root origin, but then want to drop privileges for
the rest of the program. Currently these programs drop privileges by
clearing the origin completely, but that also unnecessarily limits the
range of actions available to the rest of the program. Using
DescendOrigin or AliasOrigin allows the sending chain to instruct the
receiving chain what the deprivileged real origin is.

See https://github.com/polkadot-fellows/RFCs/pull/109 and
https://github.com/polkadot-fellows/RFCs/pull/122 for more details on
how DescendOrigin and AliasOrigin could be used instead of ClearOrigin.

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Signed-off-by: Adrian Catangiu <[email protected]>