Support searchable attributes for unstructured indexes

[papa99do]

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

• What is the current behavior? (You can also link to an open issue here)
  Searchable attributes is not supported for unstructured index since we store embeddings of all tensor fields in one field, and all string fields in one vespa field.

• What is the new behavior (if this is a feature change)?
  Searchable attributes is for unstructured index. When new documents are added to an unstructured index, new lexical and tensor fields are dynamically added to the index. This allows user to specify searchable attributes to selectively search on certain fields when using tensor, lexical or hybrid search.

• 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)
  N/A. Client will remain the same

• Related documentation changes (link commit/PR here)
  To be provided

• Other information:
  This PR also include the following changes which are required to add this feature

• A refactoring of the add_document logic
• Extracted add_document method in the base MarqoTestCase. it force refresh the index after adding docs to make sure the following get and search requests always get the latest index settings in all tests.
• Introduced a SemiStructuredMarqoIndex type and corresponding VespaIndex implementation. This type of index will be created when an UnstructuredMarqoIndexRequest is received going forward. The UnstructuredMarqoIndex will be kept so unstructured index created before this release can still work as is.

• 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)