feat(client): checkout service

[mkucmus]

Changes

WIP
part of #1138

Checklist

• the list of features is updated (if it's a feature and it's relevant)
• I followed contributing guidelines

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💙 shopware-pwa-canary successfully deployed at https://0acf0d03eca30cc516a919bc500d649ffbe4457a.shopware-pwa-canary.preview.storefrontcloud.io

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✅ mkucmus
❌ patzick
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