wstransport.nim: Rng -> ref HmacDrbgContext. 'nim-websock' did the same.

[Ivansete-status]

This PR is aimed to fix a compilation issue in the nwaku repo.

I we update the nim-websock vendor to its latest state, then the nim-libp2p doesn't compile because the Rng type is not found.

The reason why that happens is that the next commit removed the Rng type ( ref HmacDrbgContext, websock/utils.nim ) from nim-websock:

status-im/nim-websock@f8ed9b4

Therefore, this PR removes the dependency with Rng type and uses the original type: ref HmacDrbgContext.

---

This is the compilation error that happens in nwaku if nim-websock is in f8ed9b40a5ff27ad02a3c237c4905b0924e3f982 and nim-libp2p is in 41649f0:

/home/ivansete/workspace/status/nwaku/waku/waku_archive/driver/queue_driver/queue_driver.nim(271, 3) Warning: catch a more precise Exception deriving from CatchableError or Defect. [BareExcept]
/home/ivansete/workspace/status/nwaku/vendor/nim-libp2p/libp2p/transports/wstransport.nim(114, 10) Error: undeclared identifier: 'Rng'
candidates (edit distance, scope distance); see '--spellSuggest': 
 (2, 2): 'And' [enumField declared in /home/ivansete/workspace/status/nwaku/vendor/nim-libp2p/libp2p/multiaddress.nim(43, 13)]
 (2, 2): 'And' [enumField declared in /home/ivansete/workspace/status/nwaku/vendor/nim-libp2p/libp2p/multiaddress.nim(43, 13)]

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (unstable@41649f0). Click here to learn what that means.
The diff coverage is n/a.

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[Menduist]

Duplicate of #939 (not sure why github let that through)