gptel-context: Improve context management of buffers

[pabl0]

• gptel-context.el (gptel-context-add-buffer): Introduce new function for adding a buffer (object or name). This is useful for incorporating buffers from Lisp code in addition to interactive usage. (gptel-add-buffer): Create an alias (similar to gptel-add-file). (gptel-context-add): Utilize the new function. Add support for adding/removing items in the Buffer List menu. (gptel-context-remove): Add functionality to remove (entire) buffers from context (used when removing from the Buffer List).