Explicit Item and Ecommerce field selection

[dgitis]

Description & motivation

In order to prevent Google changes to the items and ecommerce records breaking the package, this PR hard-codes the fields output by the base_select macro which will prevent changes from breaking and allow users to override the macro and incorporate changes without waiting on a package update.

Resolves #281

Checklist

• [y ] I have verified that these changes work locally
• [y ] I have updated the README.md (if applicable)
• [ ] I have added tests & descriptions to my models (and macros if applicable)
• [y ] I have run dbt test and python -m pytest . to validate existing tests

[dgitis]

The ReadMe changes may not be necessary. I thought I'd found my test project which has data going back a long had a couple of fields missing, but it was an error on my part compounded by my needing to run a full refresh on my test environment which stopped me from catching the missing fields.