Add const modifier to HTTPRequest methods

Summary:
This is a backport of Core PR14006

https://github.com/bitcoin/bitcoin/pull/14006

Test Plan:

ninja check

Reviewers: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4590