add float16 support

[braingram]

Description

This PR adds float16 support to asdf. As this requires changes to the ndarray schema (part of the asdf standard) this PR currently has asdf-standard installed via git from the source branch for: asdf-format/asdf-standard#411 EDIT: a new version of asdf-standard was released

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[eslavich]

Looks good to me, just needs that final dependency update

[braingram]

Thanks! I will update this (and bring it out of draft) after asdf-standard is released.

[braingram]

Thanks @eslavich for the review. asdf-standard 1.1.0 (and 1.1.1) is out with the changes required for this PR. I bumped the required version (removing the dev requirement) and made a few minor docs fixes EDIT: moved these to #1770. If you have a chance would you give this another look?

[braingram]

The asdf-transform-schemas failure is expected until asdf-astropy can be updated for the new manifest added in asdf-transform-schemas 0.5.0