Add OAuth2LoginSpec#securityContextRepository

[ilgrosso]

Following discussion at #7222 , OAuth2LoginSpec#securityContextRepository was added.

[ilgrosso]

Travis CI failed because:

A problem occurred configuring project ':spring-security-samples-xml-gae'.
> Could not resolve all artifacts for configuration ':spring-security-samples-xml-gae:classpath'.
   > Could not resolve com.google.cloud.tools:appengine-gradle-plugin:1.3.5.
     Required by:
         project :spring-security-samples-xml-gae
      > Could not resolve com.google.cloud.tools:appengine-gradle-plugin:1.3.5.
         > Could not get resource 'https://repo.maven.apache.org/maven2/com/google/cloud/tools/appengine-gradle-plugin/1.3.5/appengine-gradle-plugin-1.3.5.pom'.
            > Could not HEAD 'https://repo.maven.apache.org/maven2/com/google/cloud/tools/appengine-gradle-plugin/1.3.5/appengine-gradle-plugin-1.3.5.pom'. Received status code 403 from server: Forbidden

which is clearly not related to the proposed changes.

[jzheaux]

Thanks, @ilgrosso! I've left some feedback inline.

Also, would you please change the format of your commit so that it looks more like:

Title

Fixes gh-7222

Where the title briefly describes what was done.

[ilgrosso]

@jzheaux please take a look again and let me know, thx

[jzheaux]

Thanks, @ilgrosso! This is now merged into master.

[ilgrosso]

Cool, thx for review @jzheaux