Add required xero source models and macros for purchase orders and purchase order line items

[DJFresh66]

Are you a current Fivetran customer?
Doug Jeffery, Modulous

What change(s) does this PR introduce?
Adds required source models and macros for Xero purchase orders and purchase order line items. These models are then used to create a purchase order model for a reporting structure in the xero package

Does this PR introduce a breaking change?

• Yes (please provide breaking change details below.)
• No - New models only, existing models are untouched

Is this PR in response to a previously created Issue

• Yes, Issue FEATURE - [Add Purchase Orders to Xero packages] dbt_xero#21 - note the issue was raised for xero package, not xero source but obviously xero source is a dependency
• No

How did you test the PR changes?

• CircleCi
• Other Using Fivetran transformations against our live Xero synch, manual tests in SQL to validate the volume and contents of the source tables versus the raw data

Select which warehouse(s) were used to test the PR

• BigQuery
• Redshift
• Snowflake
• Postgres
• Databricks
• Other (provide details below)

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Feedback

We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your feedback on our existing dbt packages or what you'd like to see next.

[fivetran-joemarkiewicz]

Hi @DJFresh66 thanks again so much for opening this PR! What you have included looks great. There is one request I have before we can move forward, and there are a few small changes I am going to apply before we merge this into main. Below are the changes I will make:

One request for you to add

• For our CircleCi testing we require a dummy csv table for the new source models you are adding. Would you be able to add two dummy csv tables to the integration_tests/seeds location. Here is a good example of what we are looking for.

Changes I will make once the seed data is added

• Apply a variable for users to enable the purchase order models. I checked and it seems only 40% of Xero users have these models. Therefore, we will want to disable them by default and allow users to enable them if they would like.
• Keep the versioning to be within dbt v1. Since the latest version of the package is compatible with dbt v1, we do not want to decrease the dbt range.

[DJFresh66]

Hi @fivetran-joemarkiewicz sorry for the delay getting back to you, I should be able to add the csvs as requested this week, cheers