feat: add support FulfillmentOrder

[krisatverbidio]

Add methods and some tests for FulfillmentOrder (fixes #197)

Most tests are implemented, but the final few are stubbed out empty.

[krisatverbidio]

@oliver006 , sorry I failed to squash these commits before the PR. my bad.

[oliver006]

Thanks for the PR - I hope to get to this in the next few days.

And no worries about not squashing the commits, we can do that later.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1cd307e) 100.00% compared to head (79c3cd6) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #216 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 45 46 +1 Lines 1995 2088 +93 ========================================= + Hits 1995 2088 +93 ``` | [Files Changed](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce) | Coverage Δ | | |---|---|---| | [fulfillment\_order.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-ZnVsZmlsbG1lbnRfb3JkZXIuZ28=) | `100.00% <100.00%> (ø)` | | | [goshopify.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-Z29zaG9waWZ5Lmdv) | `100.00% <100.00%> (ø)` | |

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[Viktoop]

hey all, any updates on this topic ?