Define consts for well known values.

[c9845]

Define types and constants for well-defined field values to aid in code editor type checking and auto-completion and in reducing back-and-forth between code and Shopify docs to check acceptable values. Since Shopify limits the values certain fields can accept, this library should do so as well!

Shopify defines acceptable values for certain fields. This defines those values, and an associated type, so that users are less likely to provide an unacceptable value. Code editors may even provide a list of acceptable values for a field for auto-completion. This is really just a code-writing improvement to prevent having to look up the acceptable values for a field in the Shopify API docs.

This should not break any existing code since the new types are all strings and string values, in existing code, will still be accepted.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0319854) 100.00% compared to head (510c8c0) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #221 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 44 44 Lines 1715 1715 ========================================= Hits 1715 1715 ``` | [Files Changed](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce) | Coverage Δ | | |---|---|---| | [draft\_order.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-ZHJhZnRfb3JkZXIuZ28=) | `100.00% <ø> (ø)` | | | [metafield.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-bWV0YWZpZWxkLmdv) | `100.00% <ø> (ø)` | | | [order.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-b3JkZXIuZ28=) | `100.00% <ø> (ø)` | | | [product.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-cHJvZHVjdC5nbw==) | `100.00% <ø> (ø)` | | | [variant.go](https://app.codecov.io/gh/bold-commerce/go-shopify/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-dmFyaWFudC5nbw==) | `100.00% <ø> (ø)` | |

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[oliver006]

This needs a rebase plz.

[c9845]

@oliver006 I rebased as requested.

[c9845]

Of note, it looks like similar work has already been done for payouts, wee PayoutStatus and the related const definitions.

The PayoutStatus type could be unexported though. It only needs to be exported to allow for creating of a PayoutStatus with something like var customPayoutStatus PayoutStatus = "custom". But this really should never be needed since the applicable values are defined by Shopify and, even in the off case someone does need a custom value, they can just provide a string to the field in the struct.