fn: Call extensions/overriding and more customization friendly docker driver (#1065)

In pure-runner and LB agent, service providers might want to set specific driver options.

For example, to add cpu-shares to functions, LB can add the information as extensions
to the Call and pass this via gRPC to runners. Runners then pick these extensions from
gRPC call and pass it to driver. Using a custom driver implementation, pure-runners can
process these extensions to modify docker.CreateContainerOptions.

To achieve this, LB agents can now be configured using a call overrider.

Pure-runners can be configured using a custom docker driver.

RunnerCall and Call interfaces both expose call extensions.

An example to demonstrate this is implemented in test/fn-system-tests/system_test.go
which registers a call overrider for LB agent as well as a simple custom docker driver.
In this example, LB agent adds a key-value to extensions and runners add this key-value
as an environment variable to the container.

api/agent/call.go

@@ -235,6 +235,14 @@ func WithContext(ctx context.Context) CallOpt {
 	}
 }
 
+// Pure runner can use this to pass an extension to the call
+func WithExtensions(extensions map[string]string) CallOpt {
+	return func(c *call) error {
+		c.extensions = extensions
+		return nil
+	}
+}
+
 // GetCall builds a Call that can be used to submit jobs to the agent.
 //
 // TODO where to put this? async and sync both call this
@@ -310,12 +318,19 @@ type call struct {
 	containerState ContainerState
 	slotHashId     string
 	isLB           bool
+
+	// LB & Pure Runner Extra Config
+	extensions map[string]string
 }
 
 func (c *call) SlotHashId() string {
 	return c.slotHashId
 }
 
+func (c *call) Extensions() map[string]string {
+	return c.extensions
+}
+
 func (c *call) RequestBody() io.ReadCloser {
 	if c.req.Body != nil && c.req.GetBody != nil {
 		rdr, err := c.req.GetBody()