Add MS Teams webhook handler

[Seb33300]

We are migrating from Slack to Microsoft Teams and we would like to continue receiving log notifications there, as this feature has been extremely useful for our workflow.

I am also planning to submit a PR on Laravel to support this handler if this PR is approved (Laravel already supports the slack handler).

The implementation is inspired from the existing SlackWebhookHandler.

This handler support an additional parameter toggleFields where we can specify the list of field we want to display using a toggle button (eg: useful for stacktrace)

Here is how it looks:

(Note the username and avatar has to be configured when creating the webhook URL)

When we click on the toggle button:

[Seb33300]

Hi @stof, Thanks for your review. I think all your comments are fixed now.

Please note that my proposal was based on the Slack code, so most of your comments are also applicable to the Slack implementation.