feat: Update code version to v2.12.0

[Eta0]

Tensorizer v2.12.0 Release

Happy #200!

This change updates the code version number and adds the release date for tensorizer v2.12.0.

It also:

• Changes the implementation of data format v5 to mark the presence of versioned headers with an optional feature flag, rather than that being a fixed requirement for all later data format versions
• Changes the metric reported by tensorizer.utils.CPUMemoryUsage.free to better reflect "free" memory by actually reporting available memory (as defined by psutil)
• Simplifies state handling for header metadata buffering during serialization
  • This really only needed two states, not three, so it has been switched away from using an enum internally to rather use a single bool to track state
• Fixes the logic used for checking if a feature flag set is out of bounds during deserialization
  • It previously checked if the feature flag set's value exceeded the value of the highest flag, however, as a set of bits, it should have been checking if it had any higher bits set than the highest valid flag bit
  • This presented no issues in previous versions as there was previously only one feature flag defined (for encryption), and the invalid logic worked anyway in the case of a single flag