feat: add function for signature verification

[brpat07]

Fixes

Use the VerifySignature function to verify signed event webhook requests.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the master branch
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[codecov]

Codecov Report

Merging #390 into master will increase coverage by 0.29%.
The diff coverage is 79.31%.

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
+ Coverage   72.57%   72.87%   +0.29%     
==========================================
  Files           4        4              
  Lines         485      505      +20     
==========================================
+ Hits          352      368      +16     
- Misses        128      130       +2     
- Partials        5        7       +2     

Impacted Files	Coverage Δ
helpers/eventwebhook/eventwebhook.go	79.31% <79.31%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9c30ae7...296af5a. Read the comment docs.

[anujs3]

LGTM (please get a review from the wonderful DX team too)

[thavanle]

lgtm

[childish-sambino]

LGTM aside from naming refactor.

[childish-sambino]

👍