update linting, dep versions, add x-conformance-class and other headmatter to schema

[philvarner]

Related Issue(s):

• n/a

Proposed Changes:

1. update linting and schema validation (this was partially-broken after the move from being in the core stac-api-spec repo)
2. add x-conformance-class and several other fields to header

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.