Use fmt library for string formatting

[f0reachARR]

Description

Abstract

In this PR, I will introduce fmt formatting library instead.
This will make JSON serialization faster.

Background

Currently, we use stringstream to format string, which is used to be a part of Context JSON.
However, stringstream is known as "slower" implementation, and this is one of bottleneck in JSON serialization.

Cite from fmt

[github-actions]

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[f0reachARR]

In some my experiments, stringstream is faster than fmt. I think something is strange.
I will investigate it.

[HansRobo]

This is marked as merged because all commits are included in #1418