Rework attachments #662

Job	Run time
Check c99 compilation	32s
Check codebase format with clang-format	8s
Build raweth transport on ubuntu-latest	14s
Run unit tests on ubuntu-latest	50s
Build Zenoh from source	6m 3s
Build and test in single thread on ubuntu-latest	39s
Test multicast and unicast fragmentation	40s
Test attachments	37s
Modular build on ubuntu-latest (1, 1, 1, 1)	38s
Modular build on ubuntu-latest (1, 1, 1, 0)	43s
Modular build on ubuntu-latest (1, 1, 0, 1)	40s
Modular build on ubuntu-latest (1, 1, 0, 0)	1m 12s
Modular build on ubuntu-latest (1, 0, 1, 1)	37s
Modular build on ubuntu-latest (1, 0, 1, 0)	57s
Modular build on ubuntu-latest (1, 0, 0, 1)	42s
Modular build on ubuntu-latest (1, 0, 0, 0)	36s
Modular build on ubuntu-latest (0, 1, 1, 1)	26s
Modular build on ubuntu-latest (0, 1, 1, 0)	28s
Modular build on ubuntu-latest (0, 1, 0, 1)	56s
Modular build on ubuntu-latest (0, 1, 0, 0)	20s
Modular build on ubuntu-latest (0, 0, 1, 1)	29s
Modular build on ubuntu-latest (0, 0, 1, 0)	23s
Modular build on ubuntu-latest (0, 0, 0, 1)	31s
Modular build on ubuntu-latest (0, 0, 0, 0)	22s
	19m 43s

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