feat: add password grant utility and fix client id and grant type

[JoaoPedroAS51]

This PR add password grant utility for providers initializePasswordGrantFlow. It uses refresh scheme.

Improve laravel passport provider to support password grant. To use it simple set grantType to password, your client id and client secret and pass username and password data to loginWith.

This also fix client id and grant type for local and refresh schemes

Usage example for laravel passport:

// Auth config
auth: {
  strategies: {
    'laravel.passport': {
       endpoints: {
         user: {
           url: '/api/auth/user'
         }
       },
       url: 'http://example.com',
       grantType: 'password',
       clientId: 'client-secret',
       clientSecret: 'client-secret'
    }
  }
}

// Login page
this.$auth.loginWith('laravel.passport', {
  data: {
    username: this.email,
    password: this.password
  }
})

This closes #600

[codecov]

Codecov Report

Merging #602 into dev will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev      #602   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines            2         2           
=========================================
  Hits             2         2           

Impacted Files	Coverage Δ
test/fixtures/basic/nuxt.config.js	100.00% <ø> (ø)

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