Fix optional request body; fix payload size check

[FabijanC]

Usage related changes

• Optional body support for some non-RPC request originally introduced in Improve UX by allowing no params for JSON-RPC requests #543 and compromised with Simplify and fix dumping #572
• Close /create_block endpoint (shardlabs/starknet-devnet-rs:0.2.0-rc.2) #598
• Error message on too big request could be more informative #505 was closed with wontfix, but this PR also resolves its problem

Development related changes

• Add method to reqwest_client property of BackgroundDevnet for POSTing requests without a body.

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