[Ovmsdev] Added Pushover and SSL client support

[Marko Juhanne]

Hello all,

I've been wanting to have a bit more configurable notification system on my
mobile phone compared to the OVMS mobile app (quiet hours, different
message priority levels, override quiet hours with emergency priority,
custom sounds, persistent notifications demanding acknowledgment etc).  I
already use Pushover as notification platform for my home automation system
and it has worked nicely for these two past years so I decided to port
Pushover client to OVMS as well.

For those who don't know, Pushover is a notification platform that has apps
for iOS and Android. They also have a desktop (actually web) application as
well. You create an account and then register your application. There's a 5
USD per platform one-time fee, which I find quite reasonable. Message limit
is 7500 messages / month / application.  So if you have multiple cars, you
could register them as individual applications.  I'm not affiliated with
the company, just a happy customer.

There are now commits for Pushover support on my fork (
https://github.com/mjuhanne/Open-Vehicle-Monitoring-System-3). It's up to
date with the main branch, and has the SWCAN commits as well. I would
appreciate if others could test it before making a pull request, since
there are now quite many changes.  I didn't want to add another http
client, so I used Mongoose. But then after enabling SSL support using
mbedssl I found out that there's this client space "struct mg_connect_opts"
that will be too small (and causes crash) if MG_ENABLE_SSL is not enabled
also in every component that uses Mongoose.

To configure Pushover, there's a new Config/Notifications page on OVMS. You
can filter which notifications and events will be relayed and select their
priority levels accordingly. I thought about using web config pluging, but
maybe we could integrate this page to configure notifications for OVMS
mobile apps as well?

Best regards,
Marko Juhanne
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