Breaking: Replace Callbacks interface by Callbacks struct (server)

[tigrannajaryan]

This continues work start in #324 for Client.

The interface has the following downsides:

• Impossible to define non-trivial default behavior. Here is an example where it was needed: Add CheckRedirect callback #269 (comment)
• Adding new callbacks requires expanding the interface, which is a breaking change for existing client users.

Getting rid of the interface and keeping just a struct for callbacks solves both problems:

• Arbitrarily complex default behavior can be now defined on the struct if the user does not provide the particular callback func.
• Adding new callback funcs is not a braking change, existing users won't be affected.

[codecov]

Codecov Report

Attention: Patch coverage is 86.27451% with 7 lines in your changes missing coverage. Please review.

Project coverage is 77.81%. Comparing base (b7beae5) to head (bf2dfbb).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
server/serverimpl.go	82.14%	3 Missing and 2 partials ⚠️
server/types/callbacks.go	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
+ Coverage   77.29%   77.81%   +0.52%     
==========================================
  Files          25       25              
  Lines        2281     2321      +40     
==========================================
+ Hits         1763     1806      +43     
+ Misses        410      408       -2     
+ Partials      108      107       -1     

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[echlebek]

Looks good to me. The change is removing a level of nesting from a lot of places in the codebase too. 👍

I do see a minor possible improvement, defaults could be defined as free standing functions and set by name in SetDefaults, which would avoid the possibility of closing over function vars by accident. But, that's really a tiny nit.

[srikanthccv]

minor doc comments 👍

[tigrannajaryan]

I do see a minor possible improvement, defaults could be defined as free standing functions and set by name in SetDefaults, which would avoid the possibility of closing over function vars by accident. But, that's really a tiny nit.

Agreed. Done.