compute: add feature flag to compute notebook block

Created by: rvantonder

When showComputeComponent is true, it displays a + Compute button on the search notebook page. I tried to use a general name, in case the flag is useful in other contexts to enable some compute component outside notebooks.

Test plan

Tested manually by toggling the setting

  "experimentalFeatures": { 
    "showComputeComponent": true,
  }