feat: async generateStateFunction and checkStateFunction

[NotEvenANeko]

generateStateFunction and checkStateFunction now support async function, fastify instance can be accessed via this.

And the generateAuthorizationUri now returns a Promise<string> due to generateStateFunction can be async, which might be a breaking change.

close #238

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Test results

❯ pnpm run test
> @fastify/oauth2@7.5.0 test
> npm run test:unit && npm run test:typescript


> @fastify/oauth2@7.5.0 test:unit
> tap

​ PASS ​ test/index.test.js 209 OK 292.476ms


                         
  🌈 SUMMARY RESULTS 🌈  
                         

Suites:   ​1 passed​, ​1 of 1 completed​
Asserts:  ​​​209 passed​, ​of 209​
​Time:​   ​622.737ms
​----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 index.js |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------

> @fastify/oauth2@7.5.0 test:typescript
> tsd

[mcollina]

Thanks for opening a PR! Can you please add the new behavior to the docs?

[NotEvenANeko]

I forgot to push the commit yesterday, docs added.

[mcollina]

Can you update the branch? There are conflicts now.

[NotEvenANeko]

fixed

[mcollina]

I'm sorry, there are more conflicts :(

[mcollina]

lgtm