IDidDocumentPublicKey controller field as optional?

We are looking to use these types in a project of ours and using the `ethr` did-method. Majority of DidDoc pubKeys do not seem to define a controller as returned by the [universal resolver](https://uniresolver.io/) and wondering if [this field](https://github.com/decentralized-identity/did-common-typescript/blob/8b035a0eb15fa755f0bfd2b08d60a87d6d095b0e/lib/IDidDocumentPublicKey.ts#L17) being optional here would make sense for the time being at least?

The default pubKeys for `ethr`:
```
{
"id": "did:ethr:0xE6Fe788d8ca214A080b0f6aC7F48480b2AEfa9a6#delegate-1",
"type": "Secp256k1VerificationKey2018",
"owner": "did:ethr:0xE6Fe788d8ca214A080b0f6aC7F48480b2AEfa9a6",
"publicKeyHex": "0295dda1dca7f80e308ef60155ddeac00e46b797fd40ef407f422e88d2467a27eb"
}
```

and for `sov` seems to be:
```
{
"id": "did:sov:builder:VbPQNHsvoLZdaNU7fTBeFx#key-1",
"type": "Ed25519VerificationKey2018",
"publicKeyBase58": "GasrpJqMo3W9KApN4Dh62cGhYG24xhwNnCgqEKEiTKe6"
}
```