Use u64 for Subscription ID

[FabijanC]

Usage related changes

• No more negative IDs (not that this was ever an explicit problem)
• The spec just says the ID should be an integer
  • Spec link
  • Started a discussion on Slack
• Precision issue in starknet.js remains, could be removed if we used u32, but u32 is not among starknet-spec types

Development related changes

• Define SubscriptionId as a type in two places:
  • json_rpc api models
  • test common utils

Checklist:

• Checked out the contribution guidelines
• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• No spelling errors - ./scripts/check_spelling.sh
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
  • Once you make the PR reviewable, please avoid force-pushing
• Updated the docs if needed - ./website/README.md
• Linked the issues resolvable by this PR - linking info
• Updated the tests if needed; all passing - execution info