Add the ability to overwrite sha and ref

[jmservera]

When there's a PR from a fork and we want to compare the tree to the base with this action, this action needs write permission into the repo, but this is only allowed from PRs within the repo. To have the write rights you can use a pull_request_target event, that will run in the base context and will give the action the right permissions when the user has it.
In such a case, the sha and ref values are from the base branch and not from the current PR, so we need to provide a way to overwrite these values too.

To allow this I modified this:

• check if there are values inside the ref and sha inputs.
• modify the github.context with the provided values, and empty the eventName field to ensure that the Snapshot takes the provided ones.
• send the context as a parameter inside the SnapshotConfig.

fixes: #51

[jmservera]

Closing as the solution proposed in ff23df9 seems cleaner.