kaiax/flatkv: Add skeleton kaiax/flatkv module

[2dvorak]

Proposed changes

• This PR adds skeleton kaiax/flatkv module, which is a wrapper for Erigon's flat KV scheme for Kaia.
• This PR is a draft PR to group changes related to the kaiax/flatkv module.
• The name for the module and the flag is subject to change.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
• I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

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Further comments

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