feat(rust/signed-doc): Contest tally, improving for proper tally flow

[Mr-Leshiy]

Description

• Modified the existing contest tally function, by making election_secret_key optional, so calculating decrypted tally result along with it's proof becomes optional as well.
• Fix double vote issue
• Return participants dictionary with their voting power and ballots references.

Related Issue(s)

Part of #732

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• 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]

📚 Docs Preview

The docs for this PR can be previewed at the following URL:

https://docs.dev.projectcatalyst.io/libs/feat/tally-dreps

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 728/728}$ | ${\color{red}Fail: 0/728}$ |