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:

• https://github.com/benchttp/engine/pull/58
• https://github.com/benchttp/engine/pull/62
• https://github.com/benchttp/engine/pull/59
• https://github.com/benchttp/engine/pull/61

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