FreeMarkerTemplateProvider: Allow Attribute Usage in sendExecuteActions Email Subject

[ravinderpayal]

Description

Right now Send Email Execution Email subject doesn't support Variable/Attribute Support as the implementation doesn't provide the subject attribute list. In fact it's calling the polymorphic sibling of send function which calls the main send function with an empty list in place subject-attribute argument.
Eventually, the process template method formats the subject line with an empty list of attributes.

Discussion

https://stackoverflow.com/questions/72058054/how-to-add-realm-name-in-custom-email-template-subject/72144000?noredirect=1#comment127470381_72144000

Motivation

The motivation comes from the fact that the current subject line is too generic and to add Realm related context to it one will need support for {{attribute}} in sendExecuteActions Email Subject

Details

We can simply provision Subject Attributes list with some documentation to express the attribute position.

One example is already here:

keycloak/services/src/main/java/org/keycloak/email/freemarker/FreeMarkerEmailTemplateProvider.java

Line 159 in bfce612

send("identityProviderLinkSubject", subjectAttrs, "identity-provider-link.ftl", attributes);

I will raise a PR shortly