Expose reader positions

[ShwethaSNayak]

Change log description
Expose Streamcut API for rust client. Streamcut represent stream to readers position in that stream segment.

Purpose of the change
Closes #466, #411

What the code does

• Add API get_streamcut() which returns StreamCut object.
• User is expected to call reader_group.get_streamcut() api periodically to get current streamcuts and use them while creating ReaderGroup.
• Stream cuts information's gets updated for every 3 sec interval.

How to verify it
Existing tests should pass

[codecov]

Codecov Report

Attention: Patch coverage is 48.64865% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 72.68%. Comparing base (f4e8244) to head (c99b0a6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
+ Coverage   64.44%   72.68%   +8.24%     
==========================================
  Files          49       50       +1     
  Lines       11810    12464     +654     
  Branches    11810    12464     +654     
==========================================
+ Hits         7611     9060    +1449     
+ Misses       4199     3404     -795     

Files	Coverage Δ
src/event/reader_group.rs	94.79% <100.00%> (ø)
src/event/reader_group_state.rs	89.74% <ø> (+0.09%)	⬆️
src/event/reader.rs	71.50% <47.22%> (-0.76%)	⬇️

... and 12 files with indirect coverage changes