Prepare Release - v2.1.0

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This is an automated PR for a new release.

Please check the following before approving:

• Changelog is accurate. The documented changes for this release are printed below.
• Any files referencing a version number. Confirm it matches the version number in the branch name.

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Release Changelog Preview

v2.1.0

Features

• Add support for Document Item category in item data source. {feat/document on plugin framework #171}
• Add support for getting file attachments of an item in item data source. {feat/document on plugin framework #171}
• Add support for getting an API Credential item's credential value in item data source. {fix: set credential as password #151}
• Add support for SSH Key Item category in item data source. {feat: add ssh key data source #158}

Fixes

• Set password to null if not set. {Secure Note resource fails to apply #173}
• Throw a better error message when item creation fails. {v2.0.0 failing to apply with example code from the docs #174}
• Improve examples and documentation. {v2.0.0 failing to apply with example code from the docs #174}