Support fetching tensors as HEX encoded binary values

[dainiusjocas]

Is your feature request related to a problem? Please describe.
It is possible to feed tensors as hex encoded binary values, but it is not possible to fetch them from Vespa in neither search results nor through document responses. This would reduce the number of bytes sent around and could be consistent with feeding.

Describe the solution you'd like
It would be nice to get tensors as hex-encoded binary values with search results (this is more important for my use cases) and through document responses by setting 'format.tensors'.

Describe alternatives you've considered
Just using the current functionality where tensor values are rendered as floats.

Additional context

[geirst]

First step: Add a new enum for https://docs.vespa.ai/en/reference/query-api-reference.html#presentation.format.tensors that renders tensor in hex format in search result.