Draft of milestones update for ODCS V1.1 upgrade. Staged as an extension for inclusion in draft ppp extension.

See #373 for more details.

Notes:

• This draft does not deprecate milestone/documents as discussed in #355
• This draft does not include a code field for cross-referencing as discussed in #373
• This draft does not include a report field for free-text reporting on milestone progress as discussed in #373
• This draft does not include an additional milestone type for commercial/financial close as discussed in ppp issue #23