Adding CodeQL security scan workflow

[shovnik]

Motivation

Follow up to issue Follow up to issue open-telemetry/oteps#144

CodeQL is GitHub's static analysis engine which scans repos for security vulnerabilities. As the project grows and we near GA it might be useful to have a workflow which checks for security vulnerabilities with every PR so we can ensure every incremental change is following best development practices. Also passing basic security checks will also make sure that there aren't any glaring issues for our users.

Changes

• This PR adds CodeQL security checks to the repo
• After every run the workflow uploads the results to GitHub. Details on the run and security alerts will show up in the security tab of this repo.

Workflow Triggers

• daily cron job at 1:30am
• workflow_dispatch (in case maintainers want to trigger a security check manually)

cc- @alolita

[codeboten]

Nice, thanks for enabling this

[lzchen]

workflow_dispatch (in case maintainers want to trigger a security check manually)

How does one trigger a security check manually with workflow_dispatch?

[codeboten]

@shovnik if you have a chance to update the branch, we could get this merged.

[shovnik]

Updated the branch. Also to trigger the workflow manually if needed, there should be a Run Workflow button in the top right of the actions tab of the repo.
https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/

[alolita]

Many thanks @shovnik