Update semantic text to default to bfloat16

[liranabn]

We need a way to index to bfloat16 for inference services that use the float element type. The most straightforward way to do this is to default to bfloat16 for all inference services that use the float element type. However, this affects scoring, so we also need a way for the user to specify the element type in case they want to use float for the most accurate possible scoring. The agreed upon solution is to add an element_type param to dense vector index_options for semantic text.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[liranabn]

PR: #144236