Refactor serializedStateManager fetchSnapshot

[dannimad]

Some refactoring aroung serializedStateManager.fetchSnapshot including:

• making a clear difference between the two flows involving whether or not we're loading using a pendinglocalstate or not instead of having both flows awkwardly merged.
• making 2 free functions to get the snapshot and version from storage depending on which api we're using (getSnapshot or getSnapshotTree)
• fusion "Fluid.Container.UseLoadingGroupIdForSnapshotFetch" with supportGetSnapshotApi since they both serve the same purpose of just choosing between getSnapshot or getSnapshotFree.

I considered writing some unit tests for the new free functions but they don't do anything special but handling undefined cases.

[anthony-murphy]

nit, destructor the object

Suggested change

	this.snapshot = {
	const { baseSnapshot, snapshotBlobs} =this.pendingLocalState;
	this.snapshot = {

[anthony-murphy]

could also make this.snapshot match pending local state in terms of property names.

[dannimad]

Good idea. I'll do that in another PR since it we will need to change those names in several places

[msfluid-bot]

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