[OPENJDK-2899]Add hooks for stage-2 build

[jhuttana]

JIRA: https://issues.redhat.com/browse/OPENJDK-2899
Could you please take a look?

I haven't created the webhook yet.

[jhuttana]

I have made the suggested changes to generate random value for secret.
Also, verified that the value is getting substituted appropriately in the buildConfig after template is instantiated.

$ oc process --parameters=true jlink-app-template
NAME                    DESCRIPTION                                                GENERATOR           VALUE
JDK_VERSION             JDK version to produce a jmods image and imagestream for                       11
APP_URI                 Application to run the jlink workflow on                                       
REF                     Git Ref to run the jlink workflow on                                           
CONTEXT_DIR             Context Dir to use for the jlink workflow                                      
GITHUB_WEBHOOK_SECRET   A secret string used to configure the GitHub webhook       expression          [a-zA-Z0-9]{16}

$  oc get bc
NAME                     TYPE     FROM         LATEST
jlink-builder-jdk-17     Docker   Dockerfile   0
jlink-s2i-jdk-17         Source   Git@master   0
multistage-buildconfig   Docker   Dockerfile   1

$ oc get -o yaml bc/jlink-s2i-jdk-17
.
.
.
triggers:
.
.
.
  - github:
      secret: REuvkipBJ3aWOFEE
    type: GitHub
status:
  lastVersion: 0

[jmtd]

Fantastic, thanks!