[3.x] Add tab Metadata to Tabs & TabContainer

[ShadyChibinko]

Implements godotengine/godot-proposals#6664

This PR adds two new methods to TabContainer & Tabs for getting and setting metadata on individual tabs.

• For Tabs, I simply mirrored the way metadata is handled in ItemList & TreeItem, and added a Variant metadata; variable to the Tab struct.
• For TabContainer, metadata is saved on the tab control itself as meta, which is in line with how methods such as set_tab_title() and set_tab_icon() work.

[akien-mga]

Thanks!