Add KeyMapper abstraction for KMS

[Zhaars]

Added KeyMapper abstraction for KMS KeyEncryptor. As our KMS KeyEncryptor needs some logic for creating KeyID from KeyContext to use it in key-related operations we need to have some abstraction over it to be able to switch key mapping logic for different strategies.

New interface KeyMapper was introduced and implemented for AWS KMS.

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)