[cli][dispatcher] New CLI commands 'watch', 'unwatch' to watch nodes.

[EskoDijk]

The watch command becomes mostly useful with log-level set to 'info' or 'debug'. Methods in OTNS.py are also added such that the Python unit test can try these commands.

To make 'watch' useful (avoid an overload of warnings), the log-level of typical watch messages is set to 'info'.

[codecov-commenter]

Codecov Report

Merging #348 (f1598b6) into main (c616148) will decrease coverage by 0.35%. The diff coverage is 2.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #348      +/-   ##
==========================================
- Coverage   49.97%   49.62%   -0.36%     
==========================================
  Files          38       38              
  Lines        4608     4633      +25     
==========================================
- Hits         2303     2299       -4     
- Misses       2123     2152      +29     
  Partials      182      182              

Impacted Files	Coverage Δ
cli/CmdRunner.go	21.88% <0.00%> (-1.26%)	:arrow_down:
cli/ast.go	66.66% <ø> (ø)
dispatcher/dispatcher.go	56.60% <0.00%> (-0.50%)	:arrow_down:
web/site/bindata.go	99.00% <ø> (-0.01%)	:arrow_down:
otnstester/OtnsTest.go	89.44% <100.00%> (ø)