MaxBuffer setting in SAML Adapters

Java Servlet Filter Adapter does not support setting MaxBuffer which is used to save POST request data. It was hard-coded with '100000' in 'SamlFilter' as follows.

https://github.com/keycloak/keycloak/blob/1.9.2.Final/adapters/saml/servlet-filter/src/main/java/org/keycloak/adapters/saml/servlet/SamlFilter.java#L139

Although it's very easy to fix the only 'SamlFilter', we should be able to set in the keycloak-saml.xml file and set across all adapters as discussed on the following mailing list.

http://lists.jboss.org/pipermail/keycloak-dev/2016-April/007060.html