Add OpenAPI client generation action

[julienduchesne]

This action will be used to generate OpenAPI clients from the various OpenAPI specs we have in the org.

It will replace the copy-pasted workflows in these repos:

• https://github.com/grafana/slo-openapi-client
• https://github.com/grafana/grafana-com-public-clients

And then, it can be used to expand the client generation feature and make it available to other repos.

[julienduchesne]

Can't request reviewers. So @grafana/platform-cat, look at this