Allow topic statistics to subscribe to multiple topics

[snehaldb90]

Currently, the SubscriberTopicStatisticsNode class subscribes to only one topic for statistics collection. So, modified it to be able to subscribe and report statistics on multiple topics.

Fixes #118

[codecov[bot]]

Codecov Report

Merging #143 into master will increase coverage by 0.30%. The diff coverage is 21.73%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   27.77%   28.07%   +0.30%     
==========================================
  Files          47       45       -2     
  Lines        1890     1884       -6     
  Branches      876      887      +11     
==========================================
+ Hits          525      529       +4     
+ Misses        559      538      -21     
- Partials      806      817      +11     

Flag	Coverage Δ
#unittests	28.07% <21.73%> (+0.30%)	:arrow_up:

Impacted Files	Coverage Δ
...pic_statistics_collector/topic_statistics_node.cpp	0.00% <0.00%> (ø)
...ics_collector/test_subscriber_topic_statistics.cpp	37.56% <7.69%> (-1.62%)	:arrow_down:
...atistics_collector/subscriber_topic_statistics.hpp	52.38% <50.00%> (+1.74%)	:arrow_up:
...src/topic_statistics_collector/parameter_utils.hpp	70.00% <0.00%> (-5.00%)	:arrow_down:
...rics_collector/msg/dummy_message__type_support.cpp
...etrics_collector/msg/dummy_message__type_support.c

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[prajakta-gokhale]

@snehaldb90 mostly looks goos to me. This change will require the e2e tests to be updated as well. Can you test them locally and add the changes to this PR so we have a green CI run?