Move context-related things to new store-api logic

[mkucmus]

Acceptance criteria:

• - store-api namespace is used in endpoints.ts insted of sales-channel-api - if it's possible
• - parsing response is adjusted properly (most of nested data.data in response does not take place anymore)
• - tests are updated
• - check if there is no place when rejection is thrown and not handled (prevent error page displaying in nuxt)