Add WebSocket RPC methods: subscribeNewHeads, unsubscribe

[FabijanC]

Usage related changes

• Introduce support for starknet_subscribeNewHeads and starknet_unsubscribe.
  • Other methods using todo!.
• Partially address JSON-RPC API 0.8.0 adaptation #613
• Address TODO in main.rs which prevented SIGINT from killing Devnet instantly in block-on-interval mode.
• Remove general RPC support, i.e. methods like starknet_getNonce or devnet_mint are no longer supported via WebSocket. Apparently they never should have been, only starknet_(un)subscribe methods should be available via this protocol.
  • Not a breaking change since this was never a part of the main branch.

Development related changes

• Socket ID and subscription ID generated randomly - should have low collision probability since they use i64.
  • Just search for rand::random() to find occurrences of this.
  • Alternatively we could generate in loop until an unused value is generated.
• Refactor JsonRpcHandler:
  • Introduce new helper function to_json_rpc_request
  • Add new helper method allows_method which checks restrictedness, relying on previously defined is_json_rpc_method_restricted
• Expand the server's api property with a sockets map.
• Parts of code related to unimplemented subscription methods is commented out. This is to prevent warnings suggesting Boxing of enum variants.
• Since general RPC support via WebSocket is removed, the corresponding tests are ignored, but a new test is introduced which asserts that the general RPC support is indeed not supported.
• The case for invalid subscription request due to TooManyBlocksBack is not tested because I couldn't make it generate 1024 blocks efficiently.

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