Add pre workflow custom hooks to run scripts before workflow execution(plan, apply, etc)

[msarvar]

Implements: #500

This PR implement functionality to run user defined scripts before any workflow is executed, the feature is based on discussion in [Feature request] Dynamically generate repo atlantis.yaml config.

Based on a comment added pre_workflow_hooks to server's repo config. Custom scripts can be declared as follows:

- id: your-repo
   pre_workflow_hooks:
   - run: ./my-script
   - run: |
        my bash script inline

To run the scripts I implemented a new PreWorkflowHooksCommandRunner interface, the interface will manage execution life cycle similar to how CommandRunner works, the difference is that it does not update PR with the outcome of the execution. The PreWorkflowHooksCommandRunner is triggered when PR is created or updated and runs before the autoplan. If PreWorkflowHooksCommandRunner fails during execution it will log the error but will not update the PR or stop autoplan from running.

I haven't updated the documentation but wanted to get the changes out to get some eyes on the PR.

[codecov]

Codecov Report

Merging #1255 (c25d57c) into master (5a1e191) will increase coverage by 0.06%.
The diff coverage is 76.05%.

@@            Coverage Diff             @@
##           master    #1255      +/-   ##
==========================================
+ Coverage   69.98%   70.05%   +0.06%     
==========================================
  Files          71       74       +3     
  Lines        5414     5540     +126     
==========================================
+ Hits         3789     3881      +92     
- Misses       1275     1303      +28     
- Partials      350      356       +6     

Impacted Files	Coverage Δ
server/events/models/models.go	78.99% <ø> (ø)
server/events/project_command_builder.go	78.92% <ø> (ø)
server/events/yaml/raw/global_cfg.go	0.00% <0.00%> (ø)
server/events/pre_workflow_hooks_command_runner.go	64.58% <64.58%> (ø)
server/events/yaml/raw/pre_workflow_step.go	64.86% <64.86%> (ø)
server/events/runtime/pre_workflow_hook_runner.go	100.00% <100.00%> (ø)
server/events/yaml/valid/global_cfg.go	89.65% <100.00%> (+0.14%)	⬆️
server/events_controller.go	56.38% <100.00%> (+0.31%)	⬆️
server/server.go	62.40% <100.00%> (+0.96%)	⬆️
... and 2 more

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[nishkrishnan]

LGTM 💯

[msarvar]

Thanks for approvals! I will add some runatlantis.io to include pre-workflow-hook example.

[ishallbethat]

how is it going ? is this PR ready for merge ?

[msarvar]

@nishkrishnan @chenrui333 I update the runatlantis.io/docs to include pre-workflow-hooks as well.

[nishkrishnan]

If you rebase, the failed test will probably resolve