x-pack/filebeat/input/cel: Add Box JWT OAuth2 provider support for CEL input

Add Box JWT server authentication support to the Filebeat CEL input as a new `auth.oauth2.provider: box` option. Box requires signing a short-lived JWT with the app's private key and exchanging it at `https://api.box.com/oauth2/token` for a bearer access token, with refresh on expiry.

This is needed to collect data from Box APIs such as events and audit logs using credentials from the Box Developer Console `config.json`. That file includes clientID, clientSecret, encrypted private key, passphrase, publicKeyID, and enterpriseID. The CEL input already supports OAuth2 providers for `azure`, `google`, and `okta`. Box JWT auth is not supported today.

Box docs: https://developer.box.com/guides/authentication/jwt/without-sdk/

corresponding beats issue: https://github.com/elastic/beats/issues/52917