added main link method on item component to use in thumbnails.

[oscar-escire]

References

Add references/links to any related issues or PRs. These may include:

• Fixes Accessibility or discoverability of bitstream download links #3384

Description

A optional param was added to ds-thumbnail component to allow open a link if a user clicks on the bitstream. Also if an item has a main bitstream, this link is used. If it does not have one assigned, it takes the first available bitstream of the item's ORIGINAL bundle

Instructions for Reviewers

List of changes in this PR:

• added a optional link parameter to thumbnail component
• added a method to get the link of the main bitstream or the first item bitstream on ItemComponent
• Updated unit test which involve thumbnail Component or Item Component to fit new changes
• Added unit test for getMainBitstream method on ItemComponent

To test this PR:

1. Open an item with bitstream and thumbnail assigned
2. Clicks on the thumbnail icon to open the link

Checklist

• My PR is created against the main branch of code (unless it is a backport or is fixing an issue specific to an older branch).
• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
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