Introduce config endpoint

[FabijanC]

Usage related changes

• Close Fetch devnet Configuration #428
  • Add endpoint GET /config for retrieving the current Devnet configuration. The response can be understood as the mapping of CLI parameters (specified and default ones) in JSON format, with some irrelevant parameters omitted. See README.md changes for an example.
• Remove endpoint GET /fork_status as its response has become a part of the new endpoint's response.

Development related changes

• port of BackgroundDevnet exposed for testing purpose.
• HttpApiHandler now including server_config property for the purpose of including it in the config response.
• Use serde(rename_all = ...) and clap(rename_all = ...) where possible.

Checklist:

• Checked the relevant parts of development docs
• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
• Updated the docs if needed, including the TODO section
• Linked the issues which this PR resolves
• Updated the tests if needed; all passing (execution info)

[qperrot]

Code tested locally with different options set.