Check HPOS feauture before quering DB for labels report

[samnajian]

Description

This PR adds support for HPOS in shipping labels report. It also fixes a bug that prevented the query to include the whole days on the borders of the query range.

Related issue(s)

https://github.com/woocommerce/woocommerce-shipping/issues/923

Steps to reproduce & screenshots/GIFs

• With trunk of this repo, make a label purchase while your test shop is using HPOS.
• Visit {site-domain}/wp-admin/admin.php?page=wc-reports&tab=wcs_labels and verify you can't see the recently purchase label in the report table.
• Now build and use this branch, visit the report page and verify the recently purchase label is viewable on the report table.

Checklist

• [ ] unit tests

• [x] changelog.txt entry added
• [x] readme.txt entry added

[samnajian]

I think this is unrelated to this PR though. But one idea that I can think of to fix this is by making sure the timestamp is int using intval() on this line : echo esc_html( get_date_from_gmt( date( 'Y-m-d H:i:s', intval( $label['created'] / 1000 ) ) ) ); WDYT?

@iyut No I didn't, how come the dates you have have decimal? basically it indicates miliseconds, didn't know we aim for that high level of precision, you're suggesting a nice idea of using intval around it.

[samnajian]

I think this is unrelated to this PR though. But one idea that I can think of to fix this is by making sure the timestamp is int using intval() on this line : echo esc_html( get_date_from_gmt( date( 'Y-m-d H:i:s', intval( $label['created'] / 1000 ) ) ) ); WDYT?

Addressed in a15b67322c24ef93f0679713c1f1a019658b360a