feat: add internal polling inbound transporter

[TimoGlastra]

Temporarily internalize polling inbound transporter to make it easier to get started with the framework.

Once we have full mediator support and the new transport system we should remove it ofc. but now it makes lives simpler just by moving it from tests and exporting it from the framework.

[codecov-commenter]

Codecov Report

Merging #323 (f7200e9) into main (ca6fb13) will increase coverage by 0.05%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##             main     #323      +/-   ##
==========================================
+ Coverage   88.28%   88.33%   +0.05%     
==========================================
  Files         220      222       +2     
  Lines        4004     4030      +26     
  Branches      465      466       +1     
==========================================
+ Hits         3535     3560      +25     
- Misses        469      470       +1     

Impacted Files	Coverage Δ
src/transport/PollingInboundTransporter.ts	95.65% <95.65%> (ø)
src/transport/index.ts	100.00% <100.00%> (ø)
src/utils/sleep.ts	100.00% <100.00%> (ø)

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