`aio rt api create` allows relpath without a leading slash

Currently users can run `aio rt api create basepath relpath ...` and it will succeed, producing an endpoint like https://adobeioruntime.net/apis/namespace/basepathrelpath

On the other hand, `wsk` requires that relpath begins with a `/`, to separate the basepath and relpath, and if users don't include it, the command produces an error:
```
error: Unable to parse api command arguments: 'relpath' must begin with '/'.
```

We should update `aio rt api create` to match this `wsk` behavior

Start at the implementation of the `aio rt api create` command and its argument validation, comparing its handling of `basepath` and `relpath` with the stated `wsk` behavior. Reproduce the command without a leading slash, then make completion require the slash and verify that the invalid form errors instead of creating a joined endpoint.

Written by the indexing model from the issue text.