Use more generic endpoint info in Push Handler Interface and Rest Api

Not all Push Handlers will provide public keys, but maybe other
information
3 jobs for pushnotifications in 3 minutes and 46 seconds (queued for 1 minute and 48 seconds)
latest
Status Job ID Name Coverage
  Build
failed #215957408
shared
build:backend

00:01:16

passed #215957407
shared
build:frontend

00:02:30

 
  Test
skipped #215957409
non-shared
test
 
Name Stage Failure
failed
build:backend Build
 use Minishlink\WebPush\WebPush;

class WebPushHandler implements PushNotificationHandlerInterface

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Checked all files in 8.078 seconds, 18.000 MB memory used
ERROR: Job failed: exit code 1