chore: Simplify github workflows and allow for required jobs

[Luca Forstner (lforst)]

simplifies worfkflows into:

• checks (everything that can run without external services)
• integration-tests (everything that needs external services like e2e tests and provider tests)
• platform-tests (workflow that triggers a pr on braintrust backend repo)

Also adds a job to the checks workflow that allows us to make the runs required (in case we want to turn on auto-merge)

[Abhijeet Prasad (AbhiPrasad)]

While you're here, you can swap out

with:
  node-version: 22

To point to https://github.com/braintrustdata/braintrust-sdk-javascript/blob/main/.tool-versions instead via node-version-file

# https://github.com/actions/setup-node
# File containing the version Spec of the version to use.  Examples: package.json, .nvmrc, .node-version, .tool-versions.
# If node-version and node-version-file are both provided the action will use version from node-version.
node-version-file: ''

Otherwise I can do it in a follow up.