[FEATURE] Add ability to GET custom endpoint

Updated customEndpoint function to accept a new parameter $method which defaults to POST.
Thanks to @rodrigoaguilera for the pull request #8 to resolve issue #7.

Bugfix of status-code check

• composer update
• fixed using AND instead of OR for status check

cleanup coding, fixed Salesforce request status for patch

• updated composer packages
• set PHP version to minimum 7.4
• updated response code check for PATCH requests according to Salesforce documentation, see https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/errorcodes.htm

Added examples, added additional header option

• updated Guzzle dependency to ^7.4
• added full example in /example folder with ddev local configuration
• added option to add additional headers like on https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/headers.htm
• updated documentation

[breaking] switched version parameter in constructor to the end

The version parameter was moved to the end of the constructor as described in the manual.

Updated documentation and return value of custom endpoint

1.0.5

updated documentation and returned full request

Added new method to use custom endpoint

It is now possible to use a custom endpoint e.g. APEX endpoint.
A special method was added to provide a custom URL part.

added describe method

thanks to nachitox

Returned message in exceptions, code cleanup

1.0.2

code cleanup, added return of message to Exceptions

set default Salesforce API version to 47

1.0.1

set default Salesforce API version to 47