[FEATURE] Support multiple values in suggestField setting

[saitho]

Is your feature request related to a problem? Please describe.
I want to use the suggest feature but need multiple fields to be considered.
The use case is searching for a product title or product id.

Describe the solution you'd like
The setting plugin.tx_solr.search.suggest.suggestField can be used to define which field is used for suggests.
This field currently only works with a single value. I'd like to be able to set multiple comma-separated values, just like queryFields.

Example:

plugin.tx_solr {
  suggest {
    showTopResults = 1
    additionalTopResultsFields = productNumber_stringS
    suggestField = spell,productNumber_stringS
  }
  search.query {
    queryFields = title^5.0, productNumber_stringS^10.0
    filter.productOnly = type:tx_myextension_products
  }
}

Target versions
12.0.5