Add option to specify a specific repo and branch

[Icohedron]

Currently this action either pulls from the upstream AdoptOpenJDK/openjdk-build repo, or pulls from the head of a PR.
I would like to have the option to specify my own repo and branch to implement comment-triggered PR builds using a custom repo and branch.

[Icohedron]

@sophia-guo any progress on this?

[Icohedron]

I noticed that the getOpenjdkBuildResource does not check out the PR ref at all when usePRRef=true, and instead uses an existing openjdk-build directory in the workspace. This can be seen by the function invocation on line 60 of builder.ts and the implementation of the function on line 108 of builder.ts

I will use this behavior as a workaround for the lack of custom repo and branch options by checking out the custom repo+branch into the openjdk-build directory in the workspace to able using this with issue-comment-triggered GitHub workflows. (i.e. implementing build functionality for adoptium/aqa-tests#2296)