Add proj and raster extensions

[gadomski]

Related to

• Peeled off from Add a GDAL-based item creator #232 to reduce its scope a bit

Description

Includes two new traits:

• Fields for getting and setting fields, including prefixed ones
• Extension for defining extension schema urls and prefixes

Checklist

Delete any checklist items that do not apply (e.g. if your change is minor, it may not require documentation updates).

• Unit tests
• Documentation, including doctests
• Git history is linear
• Commit messages are descriptive
• (optional) Git commit messages follow conventional commits
• Code is formatted (cargo fmt)
• cargo test
• Changes are added to the CHANGELOG