Give ability to send test message when creating integration

[unkrich]

Problem

There's no good way to check that an integration is working as expected today until an anomaly occurs and sends a message through.

Solution

Implement a test button similar to datasources that send a test message through to the integration to confirm it is working. Today, these integrations are slack and webhooks.

Additional Context

1. First, add test button / call to frontend. Refer to data sources for how to do this easily https://github.com/monosidev/monosi/blob/master/src/ui/src/pages/settings/Sources/components/SourcesTable.tsx#L26-L42 https://github.com/monosidev/monosi/blob/master/src/ui/src/pages/settings/Sources/components/SourcesTable.tsx#L95-L101

Implement similar in integrations: https://github.com/monosidev/monosi/blob/master/src/ui/src/pages/settings/Integrations

2. Add an endpoint in integrations to handle test, again refer to data sources https://github.com/monosidev/monosi/blob/master/src/server/handlers/datasources.py#L76-L85 https://github.com/monosidev/monosi/blob/master/src/server/handlers/init.py#L30

Do the same in: https://github.com/monosidev/monosi/blob/master/src/server/handlers/integrations.py https://github.com/monosidev/monosi/blob/master/src/server/handlers/init.py#L30

3. Add method to integrations which is responsible for creating a test message and sending it.