Add AccurateDuration and NominalDuration

[AlexandreCarlton]

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This pull request attempts to document two different types of Durations per the ISO 8601 standard:

• AccurateDuration, relating to the portion of a Duration that is context-free (that is, it only contains seconds, minutes, hours, and 24-hour days).

• NominalDuration, relating ot the portion that is dependent on the position in the calendar with respect to which the duration is being evaluated. It contains calendar years, months, weeks and days.

Please see https://github.com/graphql-java/graphql-java-extended-scalars/pull/132 for the proposed implementation in graphql-java.

[linux-foundation-easycla[bot]]

• :white_check_mark:login: AlexandreCarlton / (20a0f64086243686e3d89385730a4b0d7294a77f)
• :white_check_mark:login: AlexandreCarlton / (20a0f64086243686e3d89385730a4b0d7294a77f, dd51be9d1c3290ff78204802eab3b2e2c9222d63)

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