Update fsi serialization implementation to support dynamically size serialized blobs

[arhag]

Currently the serializer for fsi assumes each finalizer's fsi is fixed size when serialized. We want to update this to work even when each finalizer's serialized fsi can vary in size.

To preserve the optimization that we do not write out the fsi of inactive finalizers on each write, their serialized data should be written first in the file and the fsi for the active finalizers should come after.

The version for the fsi file should be bumped to version 1 so that we can still support version 0 fsi files.

For now, the fsi record for each finalizer that is being serialized does not change. That will happen in a later issue. But this should still create the infrastructure needed to upgrade a version 0 fsi struct to a version 1 fsi struct (which will be useful later) even though currently they would be two different structs that have the same exact members.