Support PAT for dhis2

[PiusKariuki]

Summary

Add a high-level, single-sentence summary of what this PR changes.

Fixes #697

Details

1. Added a personalAccessTokenField to the configuration schema for the dhis2 adaptor
2. Added a conditional, that uses the PAT to authenticate if it id provided but defaults back to username/password if it it not

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

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Review Checklist

Before merging, the reviewer should check the following items:

• Does the PR do what it claims to do?
• If this is a new adaptor, added the adaptor on marketing website ?
• Are there any unit tests?
• Is there a changeset associated with this PR? Should there be? Note that
  dev only changes don't need a changeset.
• Have you ticked a box under AI Usage?

[josephjclark]

Great, thank you @PiusKariuki

Before I can release this I need to generate release notes and bump version numbers.

You can help me with this by running pnpm changeset from the repo root and following the wizard to add a MINOR change.

pnpm changeset will generate a file with user-facing release notes. Check that in and push to this branch. It should be self-explanatory and it's hard to get wrong (you can always git reset --hard to undo any changes!)