feat: transport nim-v1.15

[rlve]

Add nim-v1.15 to transport interop tests.

[rlve]

Looks like nim is failing only against python ws:

  → Results:
    → Total: 55
    ✓ Passed: 52
    ✗ Failed: 3
    - python-v0.x x nim-v1.15 (ws, noise, mplex)

    - nim-v1.15 x python-v0.x (ws, noise, yamux)
    - nim-v1.15 x python-v0.x (ws, noise, mplex)
  → Total time: 00:08:20

  ✗ 3 test(s) failed

From what I observed the issue is that python sends observedAddr (protobuf field 4) as an empty bytes field (@[]) in its identify response. It's present in the protobuf but contains zero bytes, which causes nim to reject the message when it tries to parse empty MultiAddress.

cc: @richard-ramos