feat: add $k8s_namespace ctx variables

mentioned in commit 42876cf5

mentioned in merge request to-be-continuous/samples/php-cnb-on-kubernetes!33 (merged)

mentioned in merge request to-be-continuous/samples/php-cnb-on-kubernetes!34 (merged)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!44 (merged)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!45 (merged)

mentioned in merge request to-be-continuous/samples/php-on-kubernetes!87 (merged)

mentioned in merge request to-be-continuous/samples/php-on-kubernetes!88 (merged)

mentioned in merge request to-be-continuous/samples/maven-on-kubernetes!91 (merged)

mentioned in merge request to-be-continuous/samples/maven-on-kubernetes!92 (merged)

mentioned in merge request to-be-continuous/samples/python-on-kubernetes!36 (merged)