feat: async signingSecret on ExpressReceiver

[gmathieu]

Summary

Add async support to the signingSecret for the ExpressReceiver. This is useful when using secret managers (ex. https://github.com/googleapis/nodejs-secret-manager).

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #877 (03a3e03) into main (23b4f80) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 03a3e03 differs from pull request most recent head 59663bd. Consider uploading reports for the commit 59663bd to get more accurate results

@@            Coverage Diff             @@
##             main     #877      +/-   ##
==========================================
+ Coverage   66.08%   66.11%   +0.02%     
==========================================
  Files          13       13              
  Lines        1200     1201       +1     
  Branches      353      354       +1     
==========================================
+ Hits          793      794       +1     
  Misses        338      338              
  Partials       69       69              

Impacted Files	Coverage Δ
src/receivers/ExpressReceiver.ts	67.07% <100.00%> (+0.20%)	⬆️

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[gmathieu]

@seratch quick follow up on #875. I can do without but this would make code a lot cleaner and more robust. I imagine other receivers could benefit from this pattern as well. Thanks for your thoughts

[mwbrooks]

Hey @gmathieu 👋🏻

I saw #875. Thanks for taking the time to create that pull request! While we didn't merge it, I'm glad to see we have a tight solution for developers while not adding more maintenance to our project. Definitely appreciate your understanding with it too!

I think this pull request is a fair enhancement. I've added a few maintainers as reviewers, so that someone can take a deeper look!

[seratch]

Thanks for suggesting this enhancement! Looks great to me 👍

[mwbrooks]

Hmm... it looks like this pull request was ✅ approved but 🚫 closed before it was ⬇️ merged. 🙃

Since the branch was deleted, I'm not sure if we can restore the existing PR.

@gmathieu would you mind pushing the branch back up? It may allow you to re-open this PR. Otherwise, we'll need to open another (just reference this PR for simplicity).

[gmathieu]

@mwbrooks sorry about that. Not sure what happened

[mwbrooks]

@gmathieu No worries! I was a little confused too but it's great that GitHub was able to gracefully recover the pull request when you restored the branch. 🎉

Merged in your work. Thanks a bunch for the contribution! 🙇🏻