Move scheme to webAuthentication()

[Widcket]

⚠️ This PR contains breaking changes

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

Currently, Web Auth methods (login and logout) take a scheme parameter separately. Unlike the rest of the parameters, this one is not specific to each Web Auth method, but is something that applies to Web Auth in general (in Android). One would never specify a different value for login() than for logout().

Thus this parameter belongs to the Web Auth instance itself. webAuthentication() already takes a useCredentialsManager parameter for the same reasons. So having scheme in login() and logout() would be inconsistent, and become a potential source of confusion.

This PR moves scheme out of login() and logout(), up into webAuthentication().

Before/after (usage)

[codecov]

Codecov Report

Merging #150 (351a9a3) into ga (76af7b6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                 ga     #150   +/-   ##
=========================================
  Coverage     98.69%   98.69%           
  Complexity       75       75           
=========================================
  Files            75       75           
  Lines          1379     1380    +1     
  Branches        296      296           
=========================================
+ Hits           1361     1362    +1     
  Misses            6        6           
  Partials         12       12           

Flag	Coverage Δ
auth0_flutter	100.00% <100.00%> (ø)
auth0_flutter_android	96.52% <ø> (ø)
auth0_flutter_ios	100.00% <ø> (ø)
auth0_flutter_platform_interface	99.26% <ø> (ø)

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

Impacted Files	Coverage Δ
auth0_flutter/lib/auth0_flutter.dart	100.00% <100.00%> (ø)
auth0_flutter/lib/src/web_authentication.dart	100.00% <100.00%> (ø)

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