SemanticCacheLookup policy supports non-default Vector Search distance measures

## Feature Feature **SemanticCacheLookup policy supports non-default Vector Search distance measures** Available in Apigee `1-18-0-apigee-4` and later. A new optional`<DistanceMeasureType>` element accepts `DOT_PRODUCT_DISTANCE` (the default, and the existing behavior), `COSINE_DISTANCE`,`SQUARED_L2_DISTANCE` and `L1_DISTANCE`. The policy now compares`<Threshold>` in the direction the declared measure implies, so declaring a non-default measure requires re-tuning the threshold in the same edit. The 0-to-1 restriction on`<Threshold>` is also removed.