6cc4d59 [RPC][Mining] Add sanity checks to generatecontinuous (Cave Spectre)

Pull request description:

  ### Note
  This is built on top of #878 and requires code added in #877 and #878
  ### Problem
  `generatecontinuous` can get users in trouble by not knowing what their settings will do to their resources

  ### Solution
  Add checks in `generatecontinuous`:

  - Warn if RandomX is set to less than 4 threads.
  - Warn if Sha256D is more than one thread less than the number of cores
  - Provide ability to override the warnings with a new optional parameter
  - Disallow generatecontinuous true if already running
  - Rework the result of the command to provide a JSON status

  ### Testing
   ```
  $ veil-cli generatecontinuous true 1
  {
    "success": true,
    "algorithm": "sha256d",
    "threads": 1,
    "message": "Mining started"
  }
  $ veil-cli generatecontinuous true 1
  error code: -32603
  error message:
  Mining already active
  $ veil-cli generatecontinuous false
  {
    "success": true,
    "algorithm": "sha256d",
    "threads": 0,
    "message": "Mining stopped"
  }
  ```
  ```
  $ ./veil-cli setminingalgo randomx
  {
    "success": true,
    "message": "Mining algorithm changed from sha256d to randomx"
  }
  $ veil-cli generatecontinuous true 1
  error code: -8
  error message:
  Error: RandomX must be at least 4 threads
  ```
  ```
  $ veil-cli setminingalgo sha256d
  {
    "success": true,
    "message": "Mining algorithm changed from randomx to sha256d"
  }
  $ veil-cli generatecontinuous true 100
  error code: -8
  error message:
  Error: Available cores: 4, limit sha256d to 3 threads
  $ ./veil-cli generatecontinuous true 4 true
  {
    "success": true,
    "algorithm": "sha256d",
    "threads": 4,
    "message": "Warning: Available cores: 4, limit sha256d to 3 threads
  ```

Tree-SHA512: 01bb9d485cb7d382a4cbca6fa89424cb59bd320e50cf3364397bf0d246b6a862d48616425a9bf6960dbed50fb164a96611c208e35c9ed8878bbe80f4b1f7a5de