fix: RpcCall error not private and thread safe

[eigen-value]

This PR concerns the single issue of RpcCall error state thread safety.
It introduces getters and setters for RpcCall.error fields

[github-actions]

Memory usage change @ 87402b9

Board	flash	%	RAM for global variables	%
arduino:zephyr:unoq	🔺 +1144 - +2468	+0.06 - +0.13	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/client flash	%	examples/client RAM for global variables	%	examples/clientSSL flash	%	examples/clientSSL RAM for global variables	%	examples/hci flash	%	examples/hci RAM for global variables	%	examples/monitor flash	%	examples/monitor RAM for global variables	%	examples/server flash	%	examples/server RAM for global variables	%	examples/simple_bridge flash	%	examples/simple_bridge RAM for global variables	%	examples/test flash	%	examples/test RAM for global variables	%	examples/udp_ntp_client flash	%	examples/udp_ntp_client RAM for global variables	%
arduino:zephyr:unoq	1772	0.09	0	0.0	2036	0.1	0	0.0	1512	0.08	0	0.0	1144	0.06	0	0.0	1876	0.1	0	0.0	1444	0.07	0	0.0	1920	0.1	0	0.0	2468	0.13	0	0.0

Click for full report CSV

Board,examples/client<br>flash,%,examples/client<br>RAM for global variables,%,examples/clientSSL<br>flash,%,examples/clientSSL<br>RAM for global variables,%,examples/hci<br>flash,%,examples/hci<br>RAM for global variables,%,examples/monitor<br>flash,%,examples/monitor<br>RAM for global variables,%,examples/server<br>flash,%,examples/server<br>RAM for global variables,%,examples/simple_bridge<br>flash,%,examples/simple_bridge<br>RAM for global variables,%,examples/test<br>flash,%,examples/test<br>RAM for global variables,%,examples/udp_ntp_client<br>flash,%,examples/udp_ntp_client<br>RAM for global variables,%
arduino:zephyr:unoq,1772,0.09,0,0.0,2036,0.1,0,0.0,1512,0.08,0,0.0,1144,0.06,0,0.0,1876,0.1,0,0.0,1444,0.07,0,0.0,1920,0.1,0,0.0,2468,0.13,0,0.0