Pallet macro: `pallet::constant` should support custom type for metadata

[shaunxw]

Some native rust types cannot be used in metadata, like &'static [u8], and currently it's represented by &[u8] in metadata like in claims pallet.

pallet::constant should provide a way to configure custom types for metadata. An attribute macro like pallet::constant_type("&[u8]") might be a way to do it.

[gui1117]

#8615 should remove the need for specifying the type if I'm correct.

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[gui1117]

hopefully will be fixed by #8615

[shawntabrizi]

i believe this should be fixed now