Convert VARINT to the formatter/Using approach
Convert VARINT to the formatter/Using approach

Summary:
This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/17896 | PR17896]] [2/3]
https://github.com/bitcoin/bitcoin/pull/17896/commits/2f1b2f4ed044fe005e5a6c1b55e95822e83c16df

Depends on D8626

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8627

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