[Java][RestClient][WebClient] refactor: use CollectionFormat helper method in query generation

[Mattias-Sehlstedt]

When I previously added Json-query serialization to the RestClient and WebClient, I constructed the formatting with StringUtils.collectionToDelimitedString(values, collectionFormat.separator), this is superfluous, since CollectionFormat itself is already designed to already do exactly this with

public String collectionToString(Collection<?> collection) {
    return StringUtils.collectionToDelimitedString(collection, separator);
}

This PR changes so that the helper is properly used for the query construction.

I have also changed the CollectionFormat acccessability, since a previous PR that wanted to make everything extendable has made led to there being enum variables that are protected (which does not make sense since an enum is final).

The visibility should either be private or public, and since it a helper method that cannot be extended, I have elected to change it back to private (and that the helper method that uses the variable is public).

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[cubic-dev-ai]

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