feat: adds code coverage with coveralls

[kukkok3]

Description

Adding code coverage report for rust unit tests and flutter tests. When the other tests will be reactivated they will be added as well. Code coverage visible at https://coveralls.io/github/input-output-hk/catalyst-voices

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Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 97/97}$ | ${\color{red}Fail: 0/97}$ |

[minikin]

lgtm