Simplify the return value of the manager/connector

[brichet]

The secrets connector must currently return an object containing the namespace, the secret ID and the value.

jupyter-secrets-manager/src/token.ts

Lines 8 to 12 in 0749ddf

	export interface ISecret {
	namespace: string;
	id: string;
	value: string;
	}

This seems not necessary, it could only return a string containing the secret value. And it would simplify the implementation.

The real use of this return value seems to be for the list method.
It simplifies querying a namespace, by comparing the return value namespace to the query. We can easily use a regex for that, since the full secret id is built as namespace:id.