Implement separate "device policies" API for checking Desktop notifications

[zwass]

As discussed in #5579 (comment), this should be completed before bringing Desktop out of beta.

Currently defined /device endpoints:

fleet/server/service/handler.go

Lines 391 to 396 in d84d600

	// device-authenticated endpoints
	de := newDeviceAuthenticatedEndpointer(svc, logger, opts, r, apiVersions...)
	de.GET("/api/_version_/fleet/device/{token}", getDeviceHostEndpoint, getDeviceHostRequest{})
	de.POST("/api/_version_/fleet/device/{token}/refetch", refetchDeviceHostEndpoint, refetchDeviceHostRequest{})
	de.GET("/api/_version_/fleet/device/{token}/device_mapping", listDeviceHostDeviceMappingEndpoint, listDeviceHostDeviceMappingRequest{})
	de.GET("/api/_version_/fleet/device/{token}/macadmins", getDeviceMacadminsDataEndpoint, getDeviceMacadminsDataRequest{})

We need to add one to fetch the host's policies (+ their results, similar to how they are returned in the /api/_version_/fleet/device/{token} endpoint).

NOTE(lucas): We should expose this endpoint to be used by Go code (the Fleet Desktop tray application is made in Go: orbit/cmd/desktop/desktop.go). Should be similar to what we have for fleetctl in the server/service/client_*.go files.
Why? @sharvilshah had to copy the types to be able to make requests and parse the responses: e.g.

fleet/orbit/cmd/desktop/desktop.go

Lines 22 to 49 in 8e5cedf

	type Response struct {
	Host struct {
	Policies []struct {
	ID int `json:"id"`
	Name string `json:"name"`
	Query string `json:"query"`
	Description string `json:"description"`
	AuthorID int `json:"author_id"`
	AuthorName string `json:"author_name"`
	AuthorEmail string `json:"author_email"`
	TeamID interface{} `json:"team_id"`
	Resolution string `json:"resolution"`
	Platform string `json:"platform"`
	CreatedAt time.Time `json:"created_at"`
	UpdatedAt time.Time `json:"updated_at"`
	Response string `json:"response"`
	} `json:"policies"`
	Status string `json:"status"`
	DisplayText string `json:"display_text"`
	} `json:"host"`
	License struct {
	Tier string `json:"tier"`
	Organization string `json:"organization"`
	DeviceCount int `json:"device_count"`
	Expiration time.Time `json:"expiration"`
	Note string `json:"note"`
	} `json:"license"`
	}

[zwass]

@chiiph can you please take this over for planning as discussed in #5579? Feel free to edit the top comment description.

I've moved it into the "Prioritized" column to indicate we should try to get it ready for scheduling in the upcoming release (4.16.0).