Strongly typed context types for operation hook typings

Nobody has claimed this yet.

@bajtos [suggested to provide strongly typed context types based on the hook name](https://github.com/strongloop/loopback-datasource-juggler/pull/1820#discussion_r383908186).

For example:

```ts
export interface AccessHookContext {
Model: T;
query: Filter;
}

export type Listener = (ctx: Ctx, next: (err?: any) => void) => void;

class PersistedModel {
// ...
static observe(operation: 'access', handler: Listener): void;
static observe(operation: 'before save', handler: Listener): void;
// etc.
}
```

Start with the discussion in pull request #1820 and the existing PersistedModel.observe operation-hook typings. Identify the context type needed for each hook named by the current API, then verify that each handler receives the corresponding context and that the TypeScript declarations remain consistent across operations.

Written by the indexing model from the issue text.