Add IsSingleType to Descriptors

Summary:
IsSingleType will return whether the descriptor will give one or multiple scriptPubKeys

Backport of Core PR16528 [17/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/d1ec3e4f19487b4b100f80ad02eac063c571777d

Depends on D8413

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8415