Default schedulers and forbid trampoline as operator's scheduler

[victimsnino]

#277

[codecov]

Codecov Report

Merging #299 (8c81204) into main (5a16010) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 8c81204 differs from pull request most recent head 5a860a0. Consider uploading reports for the commit 5a860a0 to get more accurate results

@@           Coverage Diff           @@
##             main     #299   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files         109      109           
  Lines        1556     1556           
=======================================
  Hits         1529     1529           
  Misses         27       27           

Impacted Files	Coverage Δ
src/rpp/rpp/sources/from.hpp	100.00% <ø> (ø)
src/rpp/rpp/sources/interval.hpp	100.00% <ø> (ø)
src/rpp/rpp/operators/concat.hpp	100.00% <100.00%> (ø)
src/rpp/rpp/operators/merge.hpp	100.00% <100.00%> (ø)

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