fix(events,github): fixes signature validation by using raw req body

Adds raw body information (body as buffer, encoding)
to `RequestDetails` to support more request validation
use cases.

Additionally, uses the raw body to retrieve the transmitted
JSON string unparsed/raw to correctly validate the signature.

Previously, we re-stringified the parsed JSON payload
which could lead to different JSON strings.
Those differences can lead to the rejection of requests
due to a mismatch in expected signature.

Fixes: #26709
Relates-to: PR #26884
Co-authored-by: Christopher Diaz <cdiaz@rvohealth.com>
Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>

plugins/events-backend/package.json

@@ -61,6 +61,7 @@
     "@backstage/plugin-events-node": "workspace:^",
     "@backstage/types": "workspace:^",
     "@types/express": "^4.17.6",
+    "content-type": "^1.0.5",
     "express": "^4.17.1",
     "express-promise-router": "^4.1.0",
     "knex": "^3.0.0",
@@ -73,6 +74,7 @@
     "@backstage/cli": "workspace:^",
     "@backstage/plugin-events-backend-test-utils": "workspace:^",
     "@backstage/repo-tools": "workspace:^",
+    "@types/content-type": "^1.1.8",
     "supertest": "^7.0.0"
   },
   "configSchema": "config.d.ts"