Replace Apache Commons Lang 2 with Lang 3 by introducing MailTemplateManager abstraction

Replace Apache Commons Lang 2 with Lang 3 by introducing MailTemplateManager abstraction: a task in LegoFlow-SWE (Harbor dataset). AEM as a Cloud Service provides an alternative MailTemplate API that does not require StrLookup, but the codebase currently ties the two together, making it impossible…

The task

AEM as a Cloud Service provides an alternative `MailTemplate` API that does **not** require StrLookup, but the codebase currently ties the two together, making it impossible to remove Commons Lang 2 from the Cloud Service build.

Part of Lego-X/LegoFlow-SWE.