Add ability to dispatch SPL2 via SPL2 Notebooks

[fantavlik]

Building off the existing SPL Notes feature, provide new functionality to execute SPL2 as search jobs (for Splunk deployments supporting the /services/search/spl2-module-dispatch endpoint), and view results.

To try this out:

• Configure the following two Extension Settings to point to a Splunk deployment which supports /services/search/spl2-module-dispatch:
  • Splunk Rest Url
  • Token
• Open any file with the .spl2nb extension or files named modules.json.

I also drafted this Wiki page to add once this work merges: https://github.com/fantavlik/vscode-extension-splunk/wiki/Splunk-Notebooks

[abambharoliya]

@fantavlik this MR LGTM! I don't have approve permission.