Adds Spreadsheet and Document loaders

[valeksiev]

Describe the background of your pull request

The title says it all; client-lib needs to be released and this PR updated before merging.

#12

Additional context

Add any other context

Governance

• Documentation is added
• Test cases are added or updated

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• I understand that any contributions or suggestions I made may make it into the actual code. I've read the License
  https://github.com/alkem-io/Coordination/blob/master/LICENSE.
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