grid:code - leading zero for GZD (UTM Zone)

[ircwaves]

Based on the prompt here, I've read bits of the docs for the grid extension and what the NGA says, and it seems that we should inject a leading zero into the grid:code property. With the NGA saying:

For [GZD], if the UTM zone number is less than 10 in absolute value, a leading zero is preferred but not mandated. Consequently, software for information processing should accept both 5NAB123123 and 05NAB123123, for example, but should produce only 05NAB123123.

Figured I'd check here before launching a f-string tweaking PR.