Exclude semantic text subfields from field caps POC

[Mikep86]

Excludes semantic_text subfields from fields caps. Also handles if semantic_text fields have multi-fields:

PUT _inference/text_embedding/my-e5-model
{
  "service": "elasticsearch",
  "service_settings": {
    "num_allocations": 1,
    "num_threads": 1,
    "model_id": ".multilingual-e5-small"
  }
}

PUT test-field-caps
{
    "mappings": {
        "properties": {
            "test_field": {
                "type": "semantic_text",
                "inference_id": ".elser-2-elasticsearch"
            },
            "object": {
              "properties": {
                "test_field_dense": {
                    "type": "semantic_text",
                    "inference_id": "my-e5-model",
                    "fields": {
                      "text": {
                        "type": "text"
                      }
                    }
                }
              }
            },
            "non_infer_field": {
                "type": "text"
            },
            "sparse_vector": {
                "type": "sparse_vector"
            },
            "dense_vector": {
                "type": "dense_vector",
                "dims": 3,
                "similarity": "l2_norm"
            }
        }
    }
}

PUT test-field-caps/_doc/doc1
{
    "test_field": "these are not the droids you're looking for. He's free to go around",
    "object.test_field_dense": "these are not the droids you're looking for. He's free to go around",
    "sparse_vector": {
        "these": 1,
        "are": 2,
        "not": 3
    },
    "dense_vector": [1, 2, 3]
}

GET /_field_caps?allow_no_indices=true&fields=*&index=test*&ignore_unavailable=true&expand_wildcards=open

[Mikep86]

It would be nice if we could use SemanticTextField#getInferenceFieldName here, but I don't think hard-coding .inference here is a big deal overall