Added MasterKeyLoader abstraction

[Zhaars]

One of the key concepts with AcraMasterKeyLoaders(AMK) mentioned by @Lagovas is to have enough flexibility with AcraEE.

Possible scenarios could be when we want to extend some functionality related to AMKL in AcraEE(added new KMS, or have some custom cases). So this PR contains some draft ideas to have some kind of abstraction that helps us have the generic approach to creating MKL.

The key thing is to have MasterKeyLoaderFactory interface that can create any MasterKeyLoader.

type MasterKeyLoaderFactory interface {
    CreateMasterKeyLoader() (MasterKeyLoader, error)
}

MasterKeyLoaderCreator - represent the current implementation of MasterKeyLoaderFactory which depends on provided loadStrategy. So if need to have some custom behavior in AcraEE we need to reimplement the EE version of MasterKeyLoaderCreator and the remaining code should be as is.

P.S Feel free to suggest any better naming or ideas.

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)

[Lagovas]

There is open question, how we can replace existing implementation with new one? For example in acra-ee we will create new implementation in file acra/keystore/keyloader/ee_loader.go:

type EEMasterKeyLoaderCreator struct {
    loadStrategy string
    envName      string
}

func NewEEMasterKeyLoaderFactory(loadStrategy string) MasterKeyLoaderFactory {
    return EEMasterKeyLoaderCreator{
        loadStrategy: loadStrategy,
        envName:      keystore.AcraMasterKeyVarName,
    }
}

How it will replace default implementation in acra-server.go ?

[Zhaars]

There is open question, how we can replace existing implementation with new one? For example in acra-ee we will create new implementation in file acra/keystore/keyloader/ee_loader.go:

type EEMasterKeyLoaderCreator struct {
  loadStrategy string
  envName      string
}

func NewEEMasterKeyLoaderFactory(loadStrategy string) MasterKeyLoaderFactory {
  return EEMasterKeyLoaderCreator{
      loadStrategy: loadStrategy,
      envName:      keystore.AcraMasterKeyVarName,
  }
}

How it will replace default implementation in acra-server.go ?

@Lagovas Can we do a similar trick as we did with aws_enable via build flags? I mean switch on standard implementation by default but when we want to use another ee version switch off standard and switch on ee version.