Fix the validation regressions for unstructured indexes

[papa99do]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Bug fix

• What is the current behavior? (You can also link to an open issue here)

• List type of unstructured index is not validated correctly. numeric list is not captured in the validation and a generic error is raised when converting it to Vespa array type, causing the whole batch to fail
• Custom vector field not included in tensor fields will be silently ignored

• What is the new behavior (if this is a feature change)?

• Numeric list type is captured earlier in the validation phase, which only cause the individual doc to fail, with a proper error message in the response
• Custom vector field not included in tensor fields will cause the whole batch to fail.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Have unit tests been run against this PR? (Has there also been any additional testing?)
  Yes

• Related Python client changes (link commit/PR here)
  No

• Related documentation changes (link commit/PR here)
  No

• Other information:

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)

[wanliAlex]

LGTM