### Description

When completing a zero_shot classification task, there is the input sequence (provided by the user), and labels (also provided by the user). 

Tokenization as it is done now tokenizes the input sequence len(labels) times. We should really only do it once and pass those tokenized values to request builder. 

Some of the changes introduce here: https://github.com/elastic/elasticsearch/pull/84777 should make implementing this simpler (as there will be a unified temporary tokenization result object).
x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/nlp/tokenizers/BertTokenizer.java
List<String> originalVocab,
        SortedMap<String, Integer> vocab,
        boolean doLowerCase,
        boolean doTokenizeCjKChars,
        boolean doStripAccents,
        boolean withSpecialTokens,
        int maxSequenceLength,
        Set<String> neverSplit
    ) {
        this(
            originalVocab,
            vocab,
            doLowerCase,
            doTokenizeCjKChars,
            doStripAccents,
            withSpecialTokens,
            maxSequenceLength,
            Sets.union(neverSplit, NEVER_SPLIT),
            SEPARATOR_TOKEN,
            CLASS_TOKEN,
            PAD_TOKEN,
            MASK_TOKEN,
            UNKNOWN_TOKEN
        );
    }
    protected BertTokenizer(
        List<String> originalVocab,
        SortedMap<String, Integer> vocab,
        boolean doLowerCase,
        boolean doTokenizeCjKChars,