feat: moved to .net standard 2.0

[aochmann]

What this PR does / why it's submitted / why we need it:

Moved to .netstandard and added error callback.

Regarding this issue: https://github.com/thecogworks/cog-lib-configuration/issues/1

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Checklist:

• [x] My PR has a descriptive title (not a vague title like Needed this)
• [x] Title and all the commits follows commitlint guidelines
• [x] My PR targets the develop branch or appropriate release branch
• [x] I've placed the link to the corresponding Trello card or issue which this PR fixes/address below ⤵️

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Trello card: https://trello.com/c/wR7NouwI