[library usage] Refine internal token types

[bcho]

This pull request is part of the changes for #373

In this pull request, we

• refactored the TokenProvider interface to accept context.Context. This allows library user to propagate the context properly. All methods using TokenProvider are being updated as well.
• exported NewTokenProvider method later library usage.
• bump gomock to uber go mock version as the original one has been deprecated.

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (c6802f6) 64.77% compared to head (9eb41b5) 64.79%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
pkg/internal/token/interactive.go	0.00%	3 Missing ⚠️
pkg/internal/token/execCredentialPlugin.go	60.00%	2 Missing ⚠️
pkg/internal/token/federatedIdentity.go	0.00%	2 Missing ⚠️
pkg/internal/token/msi.go	0.00%	2 Missing ⚠️
pkg/internal/token/ropc.go	0.00%	2 Missing ⚠️
pkg/internal/token/serviceprincipaltoken_legacy.go	0.00%	2 Missing ⚠️
pkg/internal/token/devicecode.go	50.00%	1 Missing ⚠️
pkg/internal/token/manualtoken.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
+ Coverage   64.77%   64.79%   +0.01%     
==========================================
  Files          23       23              
  Lines        1766     1764       -2     
==========================================
- Hits         1144     1143       -1     
+ Misses        549      548       -1     
  Partials       73       73              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[weinong]

LGTM