Automatically generate SDKs

[fmvilas]

Right now, releasing SDKs is a manual process. I think we should automate this to remove the burden of having to do it manually on every release. From a conversation with @leggetter, this could look like the following:

1. Automatically trigger SDK generation when Outpost releases are published (we already have the release.yml workflow)
2. Only generate SDKs if docs/apis/openapi.yaml has changed since the last release
3. Let Speakeasy handle versioning automatically (it detects breaking vs non-breaking changes), testing and releasing

Heads up: there's presently no checking that the OpenAPI spec is correct and we have had some bugs raised. Ideally, there should be tests against a generated SDK that could guarantee the SDK (and therefore the OpenAPI spec) is correct. We've done some work already.