Make sure the string interning happens on decoding protobuf files

[ljeub-pometry]

What changes were proposed in this pull request?

The string interning logic is now always called when using the internal apis instead of being part of the high-level logic. In particular the protobuf decoder was missing it before.

Why are the changes needed?

Make sure string interning always happens.

Does this PR introduce any user-facing change? If yes is this documented?

No

How was this patch tested?

Added test for the string interning in serialise