add GraphQL::Types::ISO8601DateTime.time_precision to customize time pricisions

[gfx]

The result of Time.zone.now.to_json (Rails' datetime format) differs from GraphQL::Types::ISO8601's serialized one:

• Rails': "2010-02-02T22:30:30.124-06:00" (ISO8601 with miliseconds)
• GraphQL-Ruby's: "2010-02-02T22:30:30-06:00" (ISO8601 without milliseconds)

This incompatibility causes problems in my project 😭, so I have monkey-patched to GraphQL::Types::ISO8601 for now.

This PR provides a way to customize the precision of the serialized format just like ActiveSupport::JSON::Encoding.time_precision.

Usage:

# in a file, e.g. my_schema.rb
GraphQL::Types::ISO8601.time_precision = ActiveSupport::JSON::Encoding.time_precision

No breaking change is introduced by the PR.

[rmosolgo]

Awesome, thanks for adding this improvement!

[gfx]

👍