Update network browser logger, add mainnet URL check

[dmytrotkk]

This PR adds 2 minor changes to the network-browser module:

• Updated log format to enhance readability
• Mainnet RPC URL check at startup

New log format example:

2024-01-30 13:18:13.685 INFO    snb::loop       Running network-browser...
2024-01-30 13:18:13.686 INFO    snb::loop       SCHAIN_NAME: xxxx
2024-01-30 13:18:13.686 INFO    snb::loop       POST_ERROR_DELAY: 5000
2024-01-30 13:18:13.686 INFO    snb::loop       MULTICALL: true
2024-01-30 13:18:13.686 INFO    snb::loop       CONNECTED_ONLY: true
2024-01-30 13:18:13.686 INFO    snb::loop       NETWORK_BROWSER_TIMEOUT: 1200000
2024-01-30 13:18:13.686 INFO    snb::loop       NETWORK_BROWSER_DELAY: 10800000
2024-01-30 13:18:13.686 INFO    snb::loop       Trying to connect to the sChain RPC: xxxx
2024-01-30 13:18:14.396 INFO    snb::tools      URL is available: xxxx
2024-01-30 13:18:14.396 INFO    snb::loop       Trying to connect to the mainnet RPC: xxxx
2024-01-30 13:18:14.594 INFO    snb::tools      URL is available: xxxx
2024-01-30 13:18:14.776 INFO    snb::browser    Browse iteration started, collecting chains
2024-01-30 13:18:15.906 INFO    snb::browser    Going to gather information about 5 chains
2024-01-30 13:18:16.463 INFO    snb::tools      Going to save data to file: ./test_schainsData.json
2024-01-30 13:18:16.465 INFO    snb::tools      Successfully moved the file from ./test_schainsData.json.tmp to ./test_schainsData.json
2024-01-30 13:18:16.465 INFO    snb::browser    Browse execution time: 1689.575959 ms (1.689575959 s) for 5 chains

No performance changes.
No additional tests needed.

Fixes https://github.com/skalenetwork/internal-support/issues/912