kubeseal: client library

Is there any interest in providing a kubeseal client library? I'm specifically after the `Seal` function. The use-case is for custom applications generated SealedSecrets.

Currently [func seal](https://github.com/bitnami-labs/sealed-secrets/blob/93fbf7822abf2d3491007eae2cad997d53908f6a/cmd/kubeseal/main.go#L229) is non-exported and lives directly in `cmd/kubeseal`.

e.g. pseudo code
```go
package client

type Sealer interface {
Seal(key, Secret) (SealedSecret, error)
}
```

Start with cmd/kubeseal/main.go and the non-exported func seal linked in the issue. Trace its inputs and outputs, then determine the scope and API shape for exposing sealing to custom applications; done means a reusable client library can provide the proposed Seal operation.

Written by the indexing model from the issue text.