Extract 3rd Party Integrations into plugin system

[noahnu]

Is your feature request related to a problem? Please describe.

Fullstory, datadog, slack, pagerduty are all baked into the code. This makes sanity-runner very specific to Top Hat's tech stack. We should build a plugin system so these integrations are optional add-ons.

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A plugin system similar to how we do it in monodeploy (e.g. tappable). Fullstory/datadog etc can be first-party plugins we maintain in the sanity-runner monorepo.

[noahnu]

I've pulled out the 3rd party integrations into "plugins", however they're still hardcoded in the service at the moment.

TODO:

• Support dynamic loading of plugins.
• Move reporting plugins from service to client? Create both service and client plugins.

Unanswered questions:

• How do plugins interact with each other?