Merge bitcoin/bitcoin#24464: logging: Add severity level to logs

e11cdc930375eaec8d737e116138b2f2018c099f  logging: Add log severity level to net.cpp (klementtan)
a8290649a6578df120a71c9493acdf071e010d96 logging: Add severity level to logs. (klementtan)

Pull request description:

  **Overview**: This PR introduces a new macro, `LogPrintLevel`, that allows developers to add logs with the severity level. Additionally, it will also print the log category if it is specified.

  Sample log:
  ```
  2022-03-04T16:41:15Z [opencon] [net:debug] trying connection XX.XX.XXX.XXX:YYYYY lastseen=2.7hrs
  ```

  **Motivation**: This feature was suggested in #20576 and I believe that it will bring the following benefits:
  * Allow for easier filtering of logs in `debug.log`
  * Can be extended to allow users to select the minimum level of logs they would like to view (not in the scope of this PR)

  **Details**:
  * New log format. `... [category:level]...`. ie:
    * Do not print category if `category == NONE`
    * Do not print level if `level == NONE`
    * If `category == NONE` and `level == NONE`, do not print any fields (current behaviour)
  * Previous logging functions:
    * `LogPrintf`:  no changes in log as it calls `LogPrintf_` with `category = NONE` and `level = NONE`
    * `LogPrint`: prints additional `[category]` field as it calls `LogPrintf_` with `category = category` and `level = NONE`
  * `net.cpp`: As a proof of concept, updated logs with obvious severity (ie prefixed with `Warning/Error:..`) to use the new logging with severity.

  **Testing**:
  * Compiling and running `bitcoind` with this PR should instantly display logs with the category name (ie `net/tor/...`)
  * Grepping for `net:debug` in `debug.log` should display the updated logs with severity level:
    <details>
    <summary>Code</summary>

    ```
    $ grep "net:debug" debug.log

    2022-03-04T16:41:15Z [opencon] [net:debug] trying connection XXX:YYY lastseen=2.7hrs
    2022-03-04T16:41:16Z [opencon] [net:debug] trying connection XXX:YYY lastseen=16.9hrs
    2022-03-04T16:41:17Z [opencon] [net:debug] trying connection XXX:YYY lastseen=93.2hrs
    2022-03-04T16:41:18Z [opencon] [net:debug] trying connection XXX:YYY lastseen=2.7hrs
    ```
    </details>

ACKs for top commit:
  laanwj:
    Code review and lightly tested ACK e11cdc930375eaec8d737e116138b2f2018c099f

Tree-SHA512: 89a8c86667ccc0688e5acfdbd399aac1f5bec9f978a160e40b0210b0d9b8fdc338479583fc5bd2e2bc785821363f174f578d52136d228e8f638a20abbf0a568f

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