identity: add eos-after property

Using `num-buffers` can be unpredictable as buffer sizes are often
arbitrary (filesrc, multifilesrc, etc.). The `error-after` property on
`identity` is better but obviously reports an error afterwards. This
adds `eos-after` which does exactly the same thing but reports EOS
instead.
6 jobs for identity-eos-after in 74 minutes and 41 seconds (queued for 6 minutes and 38 seconds)
Status Job ID Name Coverage
  Preparation
passed #61331
gst indent

00:00:30

passed #61330
manifest

00:00:17

 
  Build
passed #61332
build fedora x86_64

00:02:55

 
  Test
passed #61333
check fedora

00:15:58

 
  Full Builds
passed #61334
build android arm64 api28

00:02:22

passed #61335
build cerbero fedora x86_64

00:54:59