multi-tenancy: Implements multi-tenancy

This commit implements multi-tenancy.
DB resources _agent_ and _source_ have two additional columns: username
and org_id.
The _user_ struct is passed though the context to service layer by the
auth middleware. Each service check if username is available in the
context and use it to filter resources or to verify that the resource
belongs to the user. Otherwise, it responds with 403.

The buissness logic remains unchanged.

There is no RBAC implemented in this commit (e.g. admin **sees**
everything or only the user with _admin_ role has the right to access
mutating endpoints).

Adds:
- filter by username to agent and source store.
- mappers between api and models in the service layer.

Modifies:

Agent and source store interface are changed to accept models from the
store instead of api's models.
For example:

```
type Agent interface {
    ...
    Update(ctx context.Context, agentUpdate apiAgent.AgentStatusUpdate) (*api.Agent, error)
    ...
}
```
is changed to
```
type Agent interface {
    ...
    Update(ctx context.Context, agent model.Agent) (*model.Agent, error)
    ...
}```

A consequence of this is that mapping between API and DB models is made
in the service layer with the help of `mappers`.

Signed-off-by: Cosmin Tupangiu <[email protected]>

api/v1alpha1/agent/openapi.yaml

@@ -47,6 +47,12 @@ paths:
             application/json:
               schema:
                 $ref: '../openapi.yaml#/components/schemas/Error'
+        "403":
+          description: Forbidden
+          content:
+            application/json:
+              schema:
+                $ref: '../openapi.yaml#/components/schemas/Error'
         "404":
           description: NotFound
           content:
@@ -95,6 +101,12 @@ paths:
             application/json:
               schema:
                 $ref: '../openapi.yaml#/components/schemas/Error'
+        "403":
+          description: Forbidden
+          content:
+            application/json:
+              schema:
+                $ref: '../openapi.yaml#/components/schemas/Error'
         "410":
           description: Gone
           content:

api/v1alpha1/openapi.yaml

@@ -78,6 +78,12 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+        "403":
+          description: Forbidden
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
         "404":
           description: NotFound
           content:
@@ -116,6 +122,12 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+        "403":
+          description: Forbidden
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
         "404":
           description: NotFound
           content:
@@ -209,6 +221,12 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+        "403":
+          description: Forbidden
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
         "500":
           description: Internal Server Error
           content:
@@ -248,6 +266,12 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+        "403":
+          description: Forbidden
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
         "404":
           description: NotFound
           content: