feat: Strict keyword handling in request methods

[jgraichen]

All request methods on Relation accept parameters and headers as keyword arguments. This clearly separates them from any data. They continue to take either params or data as a positional argument too, to support shorter call invocations.

This needs changes to users, for example:

get({}, {headers: ...})         ->  get(headers: ...)
head(id: 1)                     ->  head({id: 1})
delete(params, {headers: ...})  ->  delete(params, headers: ...)

These can stay:

get(params)
head(params)
delete(params)

The methods #post, #put, and #patch still take an optional first positional data/body argument:

put(body)                       ->  put(body)
post(body, {}, {headers: ...})  ->  post(body, headers: ...)
patch(body, {q: 1})             ->  post(body, params: {q: 1})

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.87%. Comparing base (e09d9b6) to head (96b9abc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   95.40%   95.87%   +0.46%     
==========================================
  Files          21       21              
  Lines         675      678       +3     
==========================================
+ Hits          644      650       +6     
+ Misses         31       28       -3     

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