Less flags for --raw

Objective:

Simplify the use of `kubeseal --raw`, especially when used to implement editor extensions.

Proposal:

Add a "context" flag to the "raw" command, so that kubeseal can figure out the `name`, `namespace` and `scope` parameters from the metadata section of an existing file (can be either a Secret or a SealedSecret object)

```
kubeseal --raw --raw-context=existing-sealed-secrets.yaml --from-file=data.txt
```

TBD: find a better name for `--raw-context`

Start by tracing the kubeseal raw command and how it currently receives name, namespace, and scope. Then inspect the metadata handling for Secret and SealedSecret inputs. Done means --raw accepts the proposed context file, derives those parameters, and still supports --from-file=data.txt.

Written by the indexing model from the issue text.