logging: Add common api for getting memory usage

Logging v2 did not support getting memory usage data. Adding this
support by creating common api for getting current and maximum
usage. Tracking of maximum usage is optional and can be enabled
using CONFIG_LOG_MEM_UTILIZATION.

Updated shell command to use common API.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>