feat: default title and body for notifications

[adityathebe]

Resolves: #533
Resvoles: #532

[adityathebe]

Will implement #532 in this PR

[adityathebe]

@moshloop I tried formatting the URLs but Shoutrrr doesn't support that at the moment (example: Slack).
And we would also need to specify whether to use markdown or HTML per service.

So, for now I've simply added raw URL in the default templates.

[moshloop]

@adityathebe

• Rather than agent_id:00000000-0000-0000-0000-000000000000 canary_id:018824e5-d0f5-246b-b111-d8ac02c98229 we should have agent: {id: 000, name: xyz), canary: {id: 000, name: canary, namespace: default}
• Add error / message to check template
• Add formatted labels
• Check uptime/latency being 0