Unstructured index distance metric fix

[vicilliar]

• 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)
  For unstructured indexes, distance metric is set to prenormalized-angular regardless of user input.

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

• Distance metric user configuration now works as expected.
• Refactored distance metric mapping (internal)
• Fixed typos (internal)

• 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?)
  In progress

• Related Python client changes (link commit/PR here)
  None

• Related documentation changes (link commit/PR here)
  None

• 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)