Remove gid_hash_ from AttachmentData (backport #416)

[mergify]

This member is computed in the constructor which is in turn called in rmw_publish. This hash is not serialized as part of the attachment.

I noticed this when profiling the multi-process iRobot benchmark, where hash_gid was taking around 23% of the running time of rmw_publish.

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This is an automatic backport of pull request #416 done by Mergify.

[mergify]

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	both modified:   rmw_zenoh_cpp/src/detail/attachment_helpers.cpp
	both modified:   rmw_zenoh_cpp/src/detail/attachment_helpers.hpp

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