Add min-height and max-height filters to TxSearch

[gagbo]

Closes: #5638

Description

Allow REST server to allow clients to query for a minimum and/or maximum height in transactions

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• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
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[gagbo]

I'm not sure if I should add doc in any spec file so I didn't check it.

I definitely did not check if my const are correct regarding GoDoc, I'll do this hopefully before monday.

I definitely did not add changelog, but I'll make it if you're okay with the key I chose

I didn't write tests, I just wanted to check the changes in my local client but it's still syncing. Any pointers for test writing is welcome

[gagbo]

I don't understand why the ci/circleci: proto test is failing though. Is that because the TxMinHeightKey and TxMaxHeightKey constants are exported ?

[alexanderbez]

ACK -- Thanks @gagbo!

WRT to proto, don't worry about this. The issue is due to the fact that the PR is from a forked remote. We should probably look into how to fix that.