[5.x] Fix field config overrides being lost when ensuring referenced fields

[el-schneider]

Blueprint field overrides were being discarded when Collection.ensureEntryBlueprintFields() called Blueprint.ensureField().

The merge logic in Blueprint.addEnsuredFieldToContents() now correctly prioritizes: fieldOverrides > referencedFieldConfig > ensuredConfig.

example:

- handle: slug
  field: common.slug
  config:
    visibility: read_only

before:

- handle: slug
  field: common.slug
  config:
    validate: 'max:200'
    localizable: true

after:

- handle: slug
  field: common.slug
  config:
    visibility: read_only
    validate: 'max:200'
    localizable: true