Add tests for subscriptions in Federation

[ardatan]

This PR adds tests to test subscriptions in @graphql-tools/federation.
It adds two subgraphs that has different fields of Post, and one of the subgraphs have a subscription field, and the nested field post is filled by another subgraph. So it tests if type merging works as expected within subscriptions.

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Latest commit: 0a1cf66

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[github-actions]

✅ Benchmark Results

     ✓ no_errors
     ✓ expected_result

     checks.........................: 100.00% ✓ 304       ✗ 0  
     data_received..................: 35 MB   3.5 MB/s
     data_sent......................: 130 kB  13 kB/s
     http_req_blocked...............: avg=43.25µs  min=2.21µs   med=3.09µs   max=6.07ms   p(90)=4.34µs   p(95)=4.62µs  
     http_req_connecting............: avg=39.46µs  min=0s       med=0s       max=5.99ms   p(90)=0s       p(95)=0s      
     http_req_duration..............: avg=61.66ms  min=51.65ms  med=57.33ms  max=165.56ms p(90)=71.66ms  p(95)=93.83ms 
       { expected_response:true }...: avg=61.66ms  min=51.65ms  med=57.33ms  max=165.56ms p(90)=71.66ms  p(95)=93.83ms 
     http_req_failed................: 0.00%   ✓ 0         ✗ 152
     http_req_receiving.............: avg=140.65µs min=108.58µs med=137.06µs max=392.76µs p(90)=157.69µs p(95)=164.64µs
     http_req_sending...............: avg=26.69µs  min=19.9µs   med=24.41µs  max=129.11µs p(90)=32.32µs  p(95)=36.5µs  
     http_req_tls_handshaking.......: avg=0s       min=0s       med=0s       max=0s       p(90)=0s       p(95)=0s      
     http_req_waiting...............: avg=61.49ms  min=51.5ms   med=57.16ms  max=165.09ms p(90)=71.49ms  p(95)=93.67ms 
     http_reqs......................: 152     15.165673/s
     iteration_duration.............: avg=65.91ms  min=55.49ms  med=61.36ms  max=176.83ms p(90)=75.52ms  p(95)=98.56ms 
     iterations.....................: 152     15.165673/s
     vus............................: 1       min=1       max=1
     vus_max........................: 1       min=1       max=1

[github-actions]

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