Add AWSKeyManager implementation

We currently have a `LocalKeyManager` implementation for `KeyManager` but other web5 SDKs have an implementation for AWS KMS

Do we want to add an AWS KMS implementation here in this repo? Or implement the it in a different repo by using the `KeyManager` interface from this repo?

If we want to implement it in this repo, where will we place it within our code (and therefore which package will it be consumed from)?
- `web5-js` has a dedicated NPM package for this named `crypto-aws-kms` ([link](https://github.com/TBD54566975/web5-js/tree/main/packages/crypto-aws-kms))
- `web5-kt` has an `AwsKeyManager` implementation within its `crypto` package ([link](https://github.com/TBD54566975/web5-kt/blob/main/crypto/src/main/kotlin/web5/sdk/crypto/AwsKeyManager.kt))