Convert blockencodings.h to new serialization framework

Summary:
This concludes backport of Core PR18112 [5/5]
https://github.com/bitcoin/bitcoin/pull/18112/commits/353f376277ad9b87e03c9ccbc1028c4b6d12e8ea

Depends on D8789

Test Plan:

ninja all check
test/functional/test_runner.py abc_p2p_compactblocks

abc_p2p_compactblocks.py has a test for blocks with more than
65536 transactions (see D1146)

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8790