Enable to configure allowed HTTP methods for subscriptions

[dan-lee]

This is a follow-up PR for #224

Not sure about the wordy option name allowedSubscriptionHttpMethods; open for suggestions ✌️

[vercel]

Deployment failed with the following error:

The most recent charge for your active payment method has failed. Please update it here: https://vercel.com/teams/graphql-helix/settings/billing.

[github-actions]

✅ Benchmark Results

     ✓ no_errors
     ✓ expected_result

     checks.........................: 100.00% ✓ 30778       ✗ 0    
     data_received..................: 3.5 MB  346 kB/s
     data_sent......................: 4.2 MB  420 kB/s
     http_req_blocked...............: avg=1.48µs   min=900ns   med=1.3µs   max=403.8µs p(90)=1.8µs    p(95)=2.2µs   
     http_req_connecting............: avg=10ns     min=0s      med=0s      max=156.9µs p(90)=0s       p(95)=0s      
   ✓ http_req_duration..............: avg=558.49µs min=407µs   med=483.8µs max=14.98ms p(90)=649.7µs  p(95)=736.14µs
       { expected_response:true }...: avg=558.49µs min=407µs   med=483.8µs max=14.98ms p(90)=649.7µs  p(95)=736.14µs
     http_req_failed................: 0.00%   ✓ 0           ✗ 15389
     http_req_receiving.............: avg=20.01µs  min=11.6µs  med=17.4µs  max=2.8ms   p(90)=23.4µs   p(95)=29.1µs  
     http_req_sending...............: avg=9.6µs    min=5.3µs   med=7.3µs   max=1.03ms  p(90)=12.7µs   p(95)=13.8µs  
     http_req_tls_handshaking.......: avg=0s       min=0s      med=0s      max=0s      p(90)=0s       p(95)=0s      
     http_req_waiting...............: avg=528.87µs min=386.1µs med=456.4µs max=14.85ms p(90)=618.92µs p(95)=706.22µs
     http_reqs......................: 15389   1538.391732/s
     iteration_duration.............: avg=643.7µs  min=477.3µs med=566.5µs max=15.47ms p(90)=743.56µs p(95)=834.34µs
     iterations.....................: 15389   1538.391732/s
     vus............................: 1       min=1         max=1  
     vus_max........................: 1       min=1         max=1