Update /internal/setRules endpoint; fix ruleset file watcher

[tohhsinpei]

Description

2 changes:

1. Follow up to this comment in Add StorageRulesManagerRegistry to handle rules files for multiple targets #4281. This PR adds support for per-resource Storage rules files on the /internal/setRules endpoint. Note that we only allow callers to overwrite the entire rules config, not update rules for a given resource. This means we can kill the updateSourceFile endpoint on the rules manager.
2. Fix the rules file watcher, which I broke in Add StorageRulesManager class for emulator #4245. We need to reread the source file every time the file watcher detects a change at the given path; otherwise we're just reloading the same contents as before. This was not caught by the integration test because of another bug there:

   firebase-tools/scripts/storage-emulator-integration/rules/manager.test.ts

   Line 102 in 089bea2

   expect(await isPermitted(opts)).to.be.false;

   
   Note opts does not contain ruleset, so this always returns false:

   firebase-tools/src/emulator/storage/rules/utils.ts

   Lines 71 to 77 in 089bea2

   	if (!opts.ruleset) {
   	EmulatorLogger.forEmulator(Emulators.STORAGE).log(
   	"WARN",
   	`Can not process SDK request with no loaded ruleset`
   	);
   	return false;
   	}

[tohhsinpei]

This is the fix for 2)

[tonyjhuang]

thanks for the fix!