fix: Fix typings for eventwebhook.d.ts PublicKey

[Cellule]

Fixes

Installing @sendgrid/eventwebhook in a project using typescript cannot properly compile because starkbank-ecdsa does not provide any typings and none are available in DefinitelyTyped project either.
Right now the only way to use the module is to ourselves provide a definition of starkbank-ecdsa OR patch-package @sendgrid/eventwebhook

I've decided to change the import statement with a minimal interface of PublicKey which should be enough to use the package properly in a typescript project.

Technically speaking, @types/node is also required because you use Buffer in the types, but a typescript project without it would be weird so I think we can let that one as-is.

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 main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[childish-sambino]

LGTM