Define all data structures and the JoinPlanCache class with a working put/lookup API backed by a simple StringMap (will be replaced with LRU and proper memory accounting later). Set up the cache as a ServiceContext decoration.

Types we need to define: CachedAccessPath, CachedInljNode, CachedJoinNode, CachedJoinPlan, JoinPlanCacheEntry, JoinPlanCacheKey and JoinPlanCache.

We can add unit tests to verify an empty cache returns nullptr on lookup, put+lookup roundtrips and put overwrites an existing entry.