Implement and expose config encoding functions

[GregoryAlbouy]

Description

Package configio exposes utilities for decoding formatted benchttp configurations (json, yaml).
We should provide a way to perform the reverse operation.

Specs will come after the following PRs are merged:

• feat: major api improvements #58
• feat: package benchttptest #62
• feat: configio #59
• feat(configio): implement Builder #61

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