After an earlier patch allowed CreateTable to create vector indexes
together with a table, in this patch we add to UpdateTable the ability
to add a new vector index to an existing table, as well as the ability
to delete a vector index from an existing table.
The implementation is inspired by DynamoDB's syntax for GSI - just like
GSI has GlobalSecondaryIndexUpdates with "Create" and "Delete" operations,
for vector indexes we have VectorIndexUpdates supporting Create and
Delete. "Update" is not yet supported - we didn't implement yet any
parameter that can be updated - but we can easily implement it in the
future.
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