Provide user control over pod label

[SaschaSchwarze0]

Labels in Kubernetes are meant to be identifying attributes for the user, see here. As of today, a user can specify labels on all our objects for example to categorize them and to later search for them using this label.

But, we are not propagating these labels further to the TaskRun or Pod. There, we only set a set of hard-coded labels including the BuildRun, Build and BuildStrategy names. For a user, it might make sense to see his labels also on the TaskRun and Pod objects to filter them based on his own categorization.