Use 72 byte dummy signatures when watching only inputs may be used

Summary:
With watching only inputs, we do not know how large the signatures
for those inputs will be as their signers may not have implemented
71 byte signatures. Thus we estimate their fees using the 72 byte
dummy signature to ensure that we pay enough fees.

This only effects fundrawtransaction when includeWatching is true.

This is a partial backport of Core PR13666 : https://github.com/bitcoin/bitcoin/pull/13666/commits/e306be742932d4ea5aca0ea4768e54b2fc3dc6a0

Depends on D4468

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4470