egister Secret Agent on startup

Summary:
Plasma NM's KDED is loaded on boot, but effectively does nothing until
the relevant plasmoid triggers the KDED.

This makes some sense for notifications and monitors, as it allows a
user to opt of a UI plasma network manager experience.

However, we want the secret agent as early in the boot as possible.
If a user is running NM, they definitely want this component active, and
if NM isn't running then registering the agent will no-op anyway.

DBus logs show that with this patch, I am retrieving my password and
a full 1500ms before org.kde.plasmanetworkmanagement.init is called and
my initial wifi
connected 3s faster than before this patch.

We could potentially move this kded to phase 0 and get us even earlier.

Test Plan:
Had a wifi with security set to "Store password for this user only"
Added dbus-monitor into my startup scripts

Logged in, wifi connected, logs showed a difference

Reviewers: Plasma, jgrulich!

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26462