Added field_boundaries_sentinel_2_p1m for Field Boundaries PV support

[cholmes]

Related Issue(s):

Closes #1026

I modeled this off of 4c920fd which didn't have any tests, so hopefully none are needed. Just adds an option for the new https://developers.planet.com/docs/planetary-variables/field-boundaries/

Proposed Changes:

For inclusion in changelog (if applicable):

1. Support for field_boundaries_sentinel_2_p1m in Subscriptions API

Not intended for changelog:

Diff of User Interface

Old behavior: No mention of field_boundaries_sentinel_2_p1m, and failure if you try to use it.

New behavior: field_boundaries_sentinel_2_p1m can be provided to a PV subscription, and it shows up as an option in the CLI.

PR Checklist:

• [] This PR is as small and focused as possible
• [] If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• [] I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• [] This PR does not break any examples or I have updated them

(Optional) @mentions for Notifications: