Allow to specify custom accounts and balances inside in-process local testing networs

[RiccardoM]

Description

This PR adds the ability to specify custom x/auth accounts and x/bank balances inside the genesis state when running CLI unit tests.

The rationale behind this is allowing custom chains that develop custom AccountI implementations to initialize the genesis state with their custom implementation as well.

closes: #XXXX

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• Wrote unit and integration tests
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• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
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[amaury1093]

lgtm, tests seem to pass still

[orijbot]

Benchmark finished. See the result: https://github.orijtech.com/benchmark/result?id=fe5b41c453244ecc8d6a7f43b5cc93da

[orijbot]

Benchmark beginning. Status page: https://github.orijtech.com/benchmark/status?commit=129267cc92b6dd505a4ac634907bf67ec6f5f47d