Cache BuildPlan in LSP

[kayagokalp]

In an offline chat with @FuelLabs/tooling, it is stated that LSP spends considerable amount of time while dealing with build plan generation as forc-pkg does not offer an in memory representation for manifest files (it expects to read them from disk). One option is refactoring forc-pkg such that we can give it a in-memory manifests instead to remove expensive I/O operations. Although this might be a good refactor in the future, we realized that we can actually cache the build plan entirely for LSP purposes as the build plan won't be changing in any type of key stroke (other than the ones being appleid to a manifest file for the current workspace/project). So we can cache it entirely and do not need to regenerate for each keystroke.

1. Generate the build plan once the project is open
2. Cache it and use it from cache for each key stroke going forward
3. Invalidate the cache if user touches a manifest file that belongs to the current list of manifest files in the cached build plan as that is the only to invalidate it.