✨ add onlyErrors option to HttpLoggingInterceptor

[techouse]

Add ability to HttpLoggingInterceptor to only log HTTP errors.

Rationale

The HttpLoggingInterceptor amazing, however, due to its verbosity the level of output can sometimes seem overwhelming. Having the ability to only show verbose logs for HTTP errors is therefore in my mind a nice feature.

Usage

Passing onlyErrors: true to the constructor will limit the logger to only log responses with an HTTP status code of 400 and above.

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This PR is a response/solution to @Guldem's #609 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.05%. Comparing base (f5a56b6) to head (53bcf2e).

Files	Patch %	Lines
...lib/src/interceptors/http_logging_interceptor.dart	95.45%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #610      +/-   ##
===========================================
+ Coverage    92.85%   93.05%   +0.19%     
===========================================
  Files           22       22              
  Lines          490      504      +14     
===========================================
+ Hits           455      469      +14     
  Misses          35       35              

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[techouse]

@Guldem would you mind throwing an eye on this?

P.S. I've also replaced all the (immutable) String concatenation and variable re-assignments with (mutable) StringBuffers for (hopefully) better performance.

[Guldem]

Nice addition! Found 1 small thing