2-legged OAuth2 using the client_credentials

[kuzmany]

This is support for client_credentials grant type added in M4 mautic/mautic#9837

Also I've removed oauth1 mentions in readme. @RCheesley probably need more cleaning.

[codecov-commenter]

Codecov Report

Merging #257 (b6e9c52) into main (9087dde) will decrease coverage by 1.12%.
The diff coverage is 0.00%.

❗ Current head b6e9c52 differs from pull request most recent head 6e03aec. Consider uploading reports for the commit 6e03aec to get more accurate results

@@             Coverage Diff              @@
##               main     #257      +/-   ##
============================================
- Coverage     51.45%   50.33%   -1.13%     
- Complexity      406      415       +9     
============================================
  Files            30       31       +1     
  Lines          1028     1051      +23     
============================================
  Hits            529      529              
- Misses          499      522      +23     

Impacted Files	Coverage Δ
lib/Auth/TwoLeggedOAuth2.php	0.00% <0.00%> (ø)

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[dennisameling]

Thanks for working on this! This works correctly in my testing 👍🏼 Just left some comments.

There's also a bunch of old references to OAuth1 in https://github.com/mautic/api-library/blob/9087ddec6400eb0c8aa43859eb7710cc9948c1ce/lib/Auth/OAuth.php - do we want to remove those in this PR on in a follow-up PR maybe?

[dennisameling]

The token isn't always valid for one hour. Mautic users can set the access token lifetime under API settings:

... in which case /oauth/v2/token will return an expires_in of, for example, 7200 seconds (2 hrs) instead of 3600 seconds (1 hr):

{
    "access_token": "TOKEN_HERE",
    "expires_in": 7200,
    "token_type": "bearer",
    "scope": null
}

[dennisameling]

In the regular OAuth provider, there's a function called getAccessTokenData() which returns an array with access_token, expires, token_type, refresh_token.

Could we have the same method for TwoLeggedOAuth2 please, to keep things consistent? It can return access_token, expires, token_type. Especially expires is interesting here, because folks can use it to store when the token expires and they need to renew it 😊

You could store the expiration time just like it was done in lib/Auth/OAuth.php 😊

api-library/lib/Auth/OAuth.php

Line 679 in 9087dde

$this->_expires = time() + $params['expires_in'];

[escopecz]

Closing in favore of #269