feat: Generic 'force_tz()'

[trevorld]

• Makes force_tz() a generic S3 method
• Current force_tz() becomes new default method force_tz.default()
• This would allow supporting tz<-() for new datetime classes (#1082).

[vspinu]

Can you add with_tz as well then?

[trevorld]

Can you add with_tz as well then?

Okay, I've added a generic S3 method with_tz() as well. The previous with_tz() is now the default method with_tz.default().

[vspinu]

Thanks!