Delete const from return type in "virtual const std::vector<double> GetRealPackedValue() const" signature in the "PlaintextImpl" class.

[HovsepPapoyan]

Constness in the return type breaks user-side move semantics (including use-cases like openfhe-rs). Please consider removing the constness.

[HovsepPapoyan]

see simplified example of the issue:

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