Add cache encryptor in keystore

[Zhaars]

This PR includes creating a new cache encryptor (SCellKeyEncryptor) for improving Keystore operation. This can help to read keys from KMS only once and store them in an encrypted way in the cache.

Distribute changed all over acra tools that use Keystore.

Checklist

• [ ] Change is covered by automated tests
• [ ] The coding guidelines are followed
• [ ] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
• [ ] CHANGELOG.md is updated (in case of notable or breaking changes)
• [ ] CHANGELOG_DEV.md is updated
• [ ] Benchmark results are attached (if applicable)
• are up-to-date (in case of API changes)