Add soft-purge option

Thanks for working on this issue @jan.stoeckler. Here is my review.

1. +++ b/fastly_add_soft_purge.patch
   @@ -0,0 +1,168 @@
   + ¶
   ...
   + ¶
   

   minor: Remove space

2. +++ b/src/EventSubscriber/AddSoftPurgeHeaders.php
   @@ -0,0 +1,85 @@
   +    $cache_control_header .= ', stale-while-revalidate=' . $config->get('stale_while_revalidate_value');
   ...
   +      $cache_control_header .= ', stale-if-error=' . $config->get('stale_if_error_value');
   

   IMHO, we should Surrogate-Control header instead of Cache-Control. This way, we don't change the default behaviour. Ref: https://docs.fastly.com/guides/tutorials/cache-control-tutorial#when-cac...

Thanks @jan.stoeckler. updating status accordingly.

Thanks for the updates @jan.stoeckler. It looks good.

Can we get this in please? It's been in RTBC for a while.

Sorry for the delay....
I believe the Fastly team were going to be maintaining this module. I was the maintainer of the D7 version, but can help out with the D8 version too.

Patch looks good, just one thing that looks odd....

+++ b/src/Form/FastlySettingsForm.php
@@ -88,6 +94,60 @@ class FastlySettingsForm extends ConfigFormBase {
+//      '#title' => $this->t('Stale if error'),

Should this be removed or uncommented?

@Leon Kessler, sure, would you mind apply patch and remove the commented line? We can issue another patch without that line, but it's not that great chance. can go as part of commit.

Okay pushed to dev.

Tried to create a 8.x-3.1 release as well, but I do not have sufficient privileges.

awesome! thank you @Leon Kessler.