Comment triggered PR builds

[smlambert]

Comment triggered PR builds (related to adoptium/TKG#109)
-- Trigger PR testing manually using comment in the git PR
-- Allow users to specify SDK (default is nightly). Users can specify releases/nightly
-- Allow users to specify test targets (sanity.system, extended.openjdk, etc)
-- Allow other values to be specified. For example:

action nightly sanity.functional xlinuxlargeheap jdk11
action releases testA,testB xlinux,osx jdk8,jdk11 hotspot,openj9

[smlambert]

@patkarns @Icohedron @yanshan24 @xfxcwynlc @xichen1

[smlambert]

In our call, we referenced an example of comment triggered workflow (autoTestPR.yml which ends up calling some python code in the TKG repo)

In a similar fashion, can we trigger some PR tests like PushOrPRtestbuild.yml from a comment trigger instead of on push or pull_request trigger.

[patkarns]

Update: we got the arguments using a python script.

Next up, we'll trigger the PR test and check for invalid parameters (like versions).

@Icohedron