Update assertion handling to support indexed assertion labels

[dkozma]

Right now, we strip the assertion indices so that stds.exif__0, stds.exif__2, stds.exif__5 all get collapsed down to stds.exif in the HashMap. This is resulting in us losing some data, since the assertion index that is last processed will overwrite the previously processed ones.

Some questions:

• If there are actions assertions across multiple labels, do we have the SDK iterate over all of them to ensure compliance with editsAndActivity?
• Do these need to be sorted before being delivered to the client?

[emensch]

Also worth calling out:

• A single instance of an assertion can have an index appended to the label. e.g. A manifest could contain a single actions assertion with the label c2pa.actions__10

[crandmck]

@adobe export issue to Jira project CAI

[github-jira-sync-bot]

:white_check_mark: Jira issue https://jira.corp.adobe.com/browse/CAI-3370 is successfully created for this GitHub issue.