In this patch we add an abstract class, "custom_index", with a validate() method.
Each CUSTOM INDEX class needs to implement a concrete subclass of custom_index
which is used to validate if this type of custom index class may be used,
and whether the optional parameters passed to it are valid.
We change the existing CUSTOM INDEX validation code to use this new mechanism.
Finally this patch implements one concrete subclass for vector index.
Before this patch, the custom index type "vector_index" was allowed,
but after this patch it gains more validation of its optional parameters
(we support 4 specific parameters, with some rules on their values).
Of course, the vector index isn't actually implemented in this patch,
we are just improving the validation of the index creation statement.
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