ci: added default values in case repo variables are not present

[Prashansa-K]

Adding some default values in timeouts extracted from repo variables.
This would unblock open-source contributions via forks where we could at least test against
open source gateway wherever possible.

All kinds of changes may not work as we would require EE and Konnect testing but things
like lib updates, doc updates, etc. can be allowed via forks.

Update: Added a go timeout for tests as Konnect tests were failing after the default 10minute marker.
Updated that to 20m for now.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.54%. Comparing base (959c522) to head (9bfde5a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1938      +/-   ##
==========================================
- Coverage   33.58%   33.54%   -0.05%     
==========================================
  Files          76       76              
  Lines        6711     6711              
==========================================
- Hits         2254     2251       -3     
- Misses       4278     4280       +2     
- Partials      179      180       +1     

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