fetch certificates

[dnldsht]

I wrote a script that fetch certificates from https://get.dgc.gov.it/v1 that can be used with the method dcc.checkSignatureWithKeysList(keys) introduced in this pull request.

If this scripts is welcome I will update the readme and give some info about the usage

[codecov[bot]]

Codecov Report

Merging #13 (9a10386) into master (1eb7214) will increase coverage by 100.00%. The diff coverage is n/a.

@@              Coverage Diff              @@
##           master       #13        +/-   ##
=============================================
+ Coverage        0   100.00%   +100.00%     
=============================================
  Files           0         3         +3     
  Lines           0        72        +72     
  Branches        0         9         +9     
=============================================
+ Hits            0        72        +72     

Impacted Files	Coverage Δ
src/rule.js	100.00% <0.00%> (ø)
src/dcc.js	100.00% <0.00%> (ø)
src/index.js	100.00% <0.00%> (ø)

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[astagi]

Hey @dnldsht your example looks great! Just few things to change.

1. In order to maintain the same style of the other examples, please use CommonJS imports

const fetch = require('node-fetch');
const { X509Certificate, PublicKey } = require('@peculiar/x509');
const crypto = require('isomorphic-webcrypto');
const fs = require('fs');

2. Add certificate.json to .gitignore
3. Add @peculiar/x509 to dev dependencies (npm install @peculiar/x509 -D)

Feel free to add notes in README alongside this PR