Export custom fields on Transaction

[fernandobrito]

Description of change

Add support to the custom_fields field on Transaction.
Update the documentation to clarify the trailing days logic.

QUESTIONS BEFORE MERGING

• custom_fields is a "JSON" (key-value pairs). This is my first time playing with taps. What is the convention for such schemaless fields? Should I export it as a JSON as well, or as a string?

Example:

• { "id": "tx_id", "custom_fields": { "a": "b" } } vs.
• { "id": "tx_id", "custom_fields": "{ \"a\": \"b\" }" }

Picking up the PR #19.
To address the issue #17.

Thanks @KAllan357 for the useful comment on #19 (comment). You were spot on right.

Manual QA steps

• Running it locally on transactions with such fields works:

Risks

Rollback steps

• revert this branch

[cmerrick]

Hi @fernandobrito, thanks for your contribution!

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[fernandobrito]

(oh no, please ignore my typo on one of the commit messages. I was planning to squash and merge anyway if that's fine, and then I can fix it)

[cmerrick]

You did it @fernandobrito!

Thank you for signing the Singer Contribution License Agreement.

[fernandobrito]

@luandy64 let me know if you have any thoughts on that 😇