fix: upload signatures through the uploadUri link

[christian2denker]

Problem

A signature is a SignatureDataEntity, not an AttachmentDataEntity. ApptiveGridClient only looked at AttachmentDataEntity when it

• looked up the uploadUri link of the field an attachment belongs to (_uploadUriLink), and
• wrote the url the server assigned back into the form data (_applyAttachmentReplacements).

So when a form offers a uploadUri link, the signature file was uploaded to the location the server assigned, while the submitted signature still pointed at the url the client had generated in createAttachment, where no file exists. A uploadUri link on the signature field itself was ignored.

Fix

• _uploadUriLink also matches a SignatureDataEntity that holds the attachment, so the field link is preferred over the form link as for attachment fields.
• _applyAttachmentReplacements also replaces the value of a SignatureDataEntity.

Tests

Three new tests in test/api_client_upload_uri_test.dart:

• the server uri is stored in the signature
• the link of the signature field is preferred over the one of the form
• a link on the signature field alone is enough (no configuration request)

dart analyze --fatal-infos is clean, all tests pass, and no line of apptive_grid_client.dart is uncovered.

Not in this PR

The upload target stays requested with {'type': 'public'} for every attachment, signatures included. That is intended.

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