In all extension openapi specs, change 'Optional Extension' to 'Extension'

[philvarner]

Related Issue(s): #203

Proposed Changes:

1. change use of "Optional Extension" to "Extension" in all open api specs. Extensions are optional by default, so the previous wording was redundant.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.