Add client name and version as request headers

[martynmjones]

Changes proposed in this Pull Request:

This PR adds two additional headers to requests sent to the WooCommerce Connect Server:

• x-client-name
• x-client-version

Detailed test instructions:

1. Checkout add/plugin-version-header on a new test site
2. Work through onboarding and establish a connection to a Google account
3. Navigate to Marketing > Google Listings & Ads and visit various sections
4. Confirm that no errors are reported and all data is loaded as expected

Changelog entry

Add - Client name and plugin version to requests

[codecov]

Codecov Report

Merging #2008 (b2303ba) into develop (afebc1e) will increase coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #2008     +/-   ##
===========================================
+ Coverage       57.9%   58.0%   +0.1%     
- Complexity      4111    4113      +2     
===========================================
  Files            456     456             
  Lines          17658   17669     +11     
===========================================
+ Hits           10224   10255     +31     
+ Misses          7434    7414     -20     

Flag	Coverage Δ
php-unit-tests	58.0% <100.0%> (+0.1%)	⬆️

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

Impacted Files	Coverage Δ
...nal/DependencyManagement/GoogleServiceProvider.php	16.9% <100.0%> (+16.9%)	⬆️
src/PluginHelper.php	74.4% <100.0%> (+8.6%)	⬆️

[eason9487]

Thanks for the work.

I also tested this PR with a local WCS and confirmed GLA requests contained the two headers. LGTM.

[eason9487]

The closing backtick is missing.