Add --level option to logcat command

[gavv]

roc-vad logcat command streams logs from driver and prints them to console.

Currently it prints all messages. It would be convenient to add an option to print messages only of specified log level and below. For example, --level=3 would print only messages with levels CRIT, ERROR, WARN, INFO, and suppress DEBUG and TRACE messages. It would be nice to accept both numeric value, like --level=3 and its string shorthand, like --level=II. (Short-hands are: CC, EE, WW, II, DD, TT).

Filtering should be done on the driver side, so that extensive debug logs are not streamed if they're not needed. Hence we need to add log level parameter to stream_logs() rpc call and handle it in rpc server inside driver.

Related files:

• rpc/driver_protocol.proto defines stream_logs() call and available log levels
• tool/cmd_logcat.cpp - implements logcat command on the client side that calls stream_logs()
• driver/driver_service.cpp - implements stream_logs() call in the rpc service