`rawissueasset` (and other issuance RPCs) should not allow issuing asset from a legacy output

[apoelstra]

In

elements/src/confidential_validation.cpp

Lines 282 to 284 in 18fadba

	// Note: This check disallows issuances in transactions with *no* witness data.
	// This can be relaxed in a future update as a HF by passing in an empty rangeproof
	// to `VerifyIssuanceAmount` instead.

we check, when issuing assets, that a transaction has witness data attached to it, which can result in confusing errors when users try to issue assets starting from legacy outputs.

In general we want to discourage legacy outputs anyway except when absolutely necessary (e.g. when using signmessage, since we do not yet have a BIP322 implementation for Elements), since they are more costly to the user, addresses have worse checksumming properties, and they are more complex for implementors to work with. Narrowing this RPC will nudge us in that direction while also replacing a confusing footgun with a clear error message.