add missingLookupHandler in InterpolatorStringLookup

[Allan-QLB]

Add a missingLookupHandler in InterpolatorStringLookup.
When resolving a variable with prefix，the InterpolatorStringLookup finds corresponding lookup by prefix in stringLookupMap，when there is no corresponding lookup for the prefix, the missingLookupHandler is used to determine whether to use default lookup. If the missingLookupHandler is null or returns true, the default lookup will be used to resolve the variable.

This allows user to control the behavior in such case. For example user can chose using default lookup as before, return null, or throw an exception.

[garydgregory]

This doesn't make sense to me. You can provide a custom default StringLookup instead of adding the complexity of an additional layer of processing.

[Allan-QLB]

This doesn't make sense to me. You can provide a custom default StringLookup instead of adding the complexity of an additional layer of processing.

There were 2 cases that would fallback to default lookup when resolving variable with a specified prefix, 1) there is no lookup for the prefix. 2）there is a lookup for the prefix but it cannot resolve this variable. This pr allows deal with the 2 cases differently. It is hard to distinguish the 2 cases in default lookup because the prefix is removed before calling default lookup.