Cross-layout caching feature flag

[kdaker]

Adds a flag to control cross-layout caching. The flag is a static member on BlueprintView. When turned on, all blueprint view instances will use the flag.

When the flag is off:

• We reset measurement caches and measurement caching behaves the same as it did with CacheTree.
• We do not cache layout nodes.

The feature flag is off by default. Because of this we turn it on for all tests that use BlueprintView.

Below is a comparison of performance tests between main and this branch with the flag turned off. Performance is not exact but close.

Test	Feature flag off w/ caching (seconds)	Main (seconds)
test_deep_and_wide_grid	1.446 avg	1.657 avg
test_repeated_layouts	0.146 avg	0.141 avg
test_row_with_one_flexible_element	0.080 avg	0.077 avg
test_deep_element_hierarchy	0.254 avg	0.245 avg
test_deep_stacks	7.646 total	8.136 total
test_incremental_state_change	0.046 avg	0.055 avg