fix: Run store.plan() only when need it.

[shuchu]

What this PR does / why we need it:

This line of code:
registry_diff, infra_diff, new_infra = store.plan(repo)

is always executed while we run "feast apply". It will call the validate() method by default which makes the option "-skip-source-validation" invalid.

Based on the original code logic, I change it to run it only when it is required. The requirement comes from:
if store._should_use_plan():

In the implementation of _should_use_plan():

def _should_use_plan(self):   
"""Returns True if plan and _apply_diffs should be used, False otherwise."""   
"""# Currently only the local provider with sqlite online store supports plan and _apply_diffs. """  
return self.config.provider == "local" and ( self.config.online_store and self.config.online_store.type == "sqlite" )`  

the "store._should_use_plan()" return True while the provider is "local" and the online store is "sqlite".

With this PR, we can alleviate the issue 3682. If a user wants to do local development, he/she may want to change the online store to a different one other than "SQLite", the action of double call "store.validate()" can be avoided.

Which issue(s) this PR fixes:
Fixes # 3682

[felixwang9817]

/lgtm