int keys: Add support for externally-managed integration keys

[mastercactapus]

Description:
This PR intends to enhance support for externally managed keys by:

• Allowing each unique "system name" a separate namespace for keys (preventing accidental collisions)
• Displaying externally managed keys in a separate section
• Requiring an extra confirmation when deleting an externally managed key

Which issue(s) this PR fixes:
Closes #3716

Screenshots:

Describe any introduced user-facing changes:

• Integration key list now displays external keys in a separate section
• Deleting an external key requires an extra confirmation

Describe any introduced API changes:

• Creating an integration key allows setting an optional externalSystemName
• IntegrationKey type now has externalSystemName as a field

Additional Info:
Creating a key for validation can only be done through the API /api/graphql/explore

mutation createKey{
  createIntegrationKey(input:{
    name: "Foobar",
    serviceID: "72a85097-3601-43fe-a247-c0ee978333fd"
    type: generic
    externalSystemName: "Yet Another Application"
  }){
    id
  }
}

[allending313]

LGTM, tests passing and validated locally