Return const ref to reduce copies/allocations

[mjcarroll]

Found when running memory audits on the rmw implementations: there is a place where a copy is being used when a const ref should suffice, causing some additional allocations and frees:

 malloc  (not expected) 144 -> 0x5603f5f0c050
 malloc  (not expected) 24 -> 0x5603f9bf3d30
 malloc  (not expected) 144 -> 0x5603f8e5f720
 malloc  (not expected) 24 -> 0x5603f8e50180
 malloc  (not expected) 24 -> 0x5603f500ddf0
 malloc  (not expected) 38 -> 0x5603f51b01a0
 malloc  (not expected) 20 -> 0x5603f951bb20
 free    (not expected) 0x5603f951bb20
 free    (not expected) 0x5603f51b01a0
 free    (not expected) 0x5603f500ddf0
 free    (not expected) 0x5603f8e50180
 free    (not expected) 0x5603f8e5f720
 free    (not expected) 0x5603f9bf3d30
 free    (not expected) 0x5603f5f0c050

In https://github.com/eProsima/Fast-RTPS/blob/master/src/cpp/publisher/PublisherImpl.cpp#L128, when the RTPSParticipantAttributes are retrieved, a copy is returned, rather than a reference or const reference.

As far as I could tell, the copy variant of the method wasn't being used anywhere, so switching to the const ref version shouldn't cause issues. If it is being used, then having multiple variations should be suitable.

[wjwwood]

@dejanpan FYI

[MiguelCompany]

This was already done as part of #254 (commit 22ac8c9) and will be part of the next release (see #304)