Fixes for custom schemas

[braingram]

Description

Fix a few issues for custom schemas.

The main issue fixed (where custom schema is not used for write_to) is fixed by changing schema.validate to use a custom schema if one exists for the provided AsdfFile instance passed in as the ctx argument (Fixes: #1860).

The use of AsdfFile._custom_schema in schema.validate has the benefit of removing a duplicate validation that occurs when a custom schema is provided (Fixes: #1899).

The weldx downstream failures are unrelated (and appear to be due to a pint-xarray update): BAMWelDX/weldx#990

I also updated the jwst downstream tests to run serially since they are running into crds reference file download conflicts.

Tasks

• run pre-commit on your machine
• run pytest on your machine
• Does this PR add new features and / or change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update relevant docstrings and / or docs/ page
  • for any new features, add unit tests

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: bug fix
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change

[nden]

LGTM