Add weak references #876

Job	Run time
Check c99 compilation	10s
Check codebase format with clang-format	5s
Build raweth transport on ubuntu-latest	15s
Run unit tests on ubuntu-latest	31s
Build Zenoh from source	5m 30s
Build and test in single thread on ubuntu-latest	41s
Test attachments	35s
Test multicast and unicast fragmentation	41s
Modular build on ubuntu-latest (1, 1, 1, 1)	38s
Modular build on ubuntu-latest (1, 1, 1, 0)	38s
Modular build on ubuntu-latest (1, 1, 0, 1)	37s
Modular build on ubuntu-latest (1, 1, 0, 0)	34s
Modular build on ubuntu-latest (1, 0, 1, 1)	48s
Modular build on ubuntu-latest (1, 0, 1, 0)	33s
Modular build on ubuntu-latest (1, 0, 0, 1)	42s
Modular build on ubuntu-latest (1, 0, 0, 0)	35s
Modular build on ubuntu-latest (0, 1, 1, 1)	26s
Modular build on ubuntu-latest (0, 1, 1, 0)	21s
Modular build on ubuntu-latest (0, 1, 0, 1)	31s
Modular build on ubuntu-latest (0, 1, 0, 0)	23s
Modular build on ubuntu-latest (0, 0, 1, 1)	24s
Modular build on ubuntu-latest (0, 0, 1, 0)	20s
Modular build on ubuntu-latest (0, 0, 0, 1)	33s
Modular build on ubuntu-latest (0, 0, 0, 0)	22s
	16m 53s

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