feat: add basic authentication

[jludwig]

What

This pull request allows one to authenticate via basic auth, like so:

$client = new ConfluenceClient('https://example.atlassian.net'); $client->authenticateBasicAuth('example@example.com', 'password');

Why

Some Confluence instances do not allow users to create API tokens. Adding basic authentication allows users that can't (or prefer not to) use API tokens to use this PHP client.

[astepin]

Thank you for your changes. I am preparing a change where you can use BasicAuth via the address (https://myuser:passwordhere@example.atlassian.net)
This is already included in main. I will make a new release soon.