Add author_text and original source to template rest api.

[jorgefilipecosta]

This PR adds the fields author_text and original source to the template rest API.
Backports into the core WordPress/gutenberg#56333, WordPress/gutenberg#56567, and WordPress/gutenberg#56990.

Ticket: https://core.trac.wordpress.org/ticket/60358

[github-actions]

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[jorgefilipecosta]

Committed at 400a7ac.