[TASK] Populate id in event streams

[eliashaeussler]

This PR adds a new method getId() to both event streams. With these methods in place, event streams now populate the internal stream id.

[codeclimate]

Code Climate has analyzed commit 9e6eab0 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (0.0% change).

View more on Code Climate.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2060f28) 100.00% compared to head (9e6eab0) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##                main        #5   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        37        39    +2     
===========================================
  Files              6         6           
  Lines            109       113    +4     
===========================================
+ Hits             109       113    +4     

Files Changed	Coverage Δ
src/Stream/Psr7EventStream.php	100.00% <100.00%> (ø)
src/Stream/SelfEmittingEventStream.php	100.00% <100.00%> (ø)

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