Implement KMS AWS master key loading

[Zhaars]

Implementation of KMS AWS MasterKey Loading.

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)
• Example projects and code samples are up-to-date (in case of API changes)

[Lagovas]

hm, it push me to think about registries. what about to use here registries approach as we did it for CryptoHandlers? Turn implementation on with build tag and register it at start? for example:

type EncryptorFactoryMethod interface {
  New(credentialPath string) (kms.Encryptor, error)
}
// or type EncryptorFactoryMethod func(credentialPath string) (kms.Encryptor, error)
var lock = sync.Lock{}
var encryptors map[string]EncryptorFactoryMethod
RegisterEncryptor(encryptor EncryptorFactoryMethod){
  lock.Lock()
  encryptors[encryptor.ID()] = encryptor
  lock.Unlock()
}

Then we can use :

func NewLoader(credentialPath, keyIdentifierURI string) (*Loader, error) {
	keyID, err := kms.NewKeyIdentifierFromURI(keyIdentifierURI)
	if err != nil {
		return nil, err
	}
        encryptorFactory, ok := encryptors[keyID]
        if !ok {
            log.Errorln("Unknown key ID")
        }
        encryptor, err := encryptorFactory.New(credentialPath) // or encryptorFactory(credentialPath)
}

Then our AWS implementation should have:

func init(){
  kms.RegisterEncryptor(someFactoryFunction)
}

With such approach we can extend with enterprise KMS support.

Additionally, we can make default loader that do nothing with master key (current approach where env variable store result master key. this implementation will return master key as is in Encrypt/Decrypt methods) and registered by default and +1 with AWS. Then current loader will be consistent with new approach

And +1 pros for that it be able to cut AWS support and all dependencies via build tags even in CE version. If client doesn't need AWS KMS support, it can compile it with build tag that turns off it. Result binary size and amount of dependencies will be less.

[Lagovas]

lgtm

[Lagovas]

lgtm