Grant API auth before MC connect

[mikkamp]

Changes proposed in this Pull Request:

This PR removes the requirement to connect the Merchant Center account first, before being able to grant the WPCOM API authentication. The following steps were implemented in this PR:

• Remove Merchant Center ID when requesting redirect URL
• Adjust Connection Test to support granting auth permissions
• Adjust unit tests to support the same

Detailed test instructions:

Additional details:

Project Thread: pcTzPl-2FJ-p2

Changelog entry

• Tweak - Grant API auth before MC connect.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 35 lines in your changes missing coverage. Please review.

Project coverage is 67.1%. Comparing base (b04c648) to head (0519618).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/ConnectionTest.php	0.0%	35 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             develop   #2794      +/-   ##
============================================
+ Coverage       66.7%   67.1%    +0.4%     
- Complexity         0    4686    +4686     
============================================
  Files            316     480     +164     
  Lines           4921   19601   +14680     
  Branches        1204       0    -1204     
============================================
+ Hits            3282   13159    +9877     
- Misses          1502    6442    +4940     
+ Partials         137       0     -137     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	67.1% <16.7%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/API/Google/Middleware.php	95.7% <100.0%> (ø)
src/API/WP/OAuthService.php	100.0% <100.0%> (ø)
src/ConnectionTest.php	0.0% <0.0%> (ø)

... and 793 files with indirect coverage changes