Add integration for Jiff date/time types

[sgoll]

Description

This adds Jiff integration, behind feature flag jiff, for the following date/time types:

• civil::Date to GraphQL scalar LocalDate
• civil::Time to GraphQL scalar LocalTime
• civil::DateTime to GraphQL scalar LocalDateTime
• Timestamp to GraphQL scalar DateTime
• Span to GraphQL scalar Duration

This does not add integration for Zoned because there is no matching GraphQL scalar yet. See #1270 (comment) and related comments for details.

Closes #1270

[sgoll]

With 9acbe24, this is now ready for review.

[LegNeato]

I'll take a look in the next couple of days if @tyranron doesn't get to it

[tyranron]

@sgoll wow, thank you for the very thoughtful and pedantic PR!

Reviewing it was really a pleasure. Everything is almost perfect. The only few things I've adjusted:

• removed default features of jiff crate;
• removed rustdoc intra-links from GraphQL docs of the added scalars.