add configurable option to set the user agent string

[aaronpk]

aaronpk/indielogin.com#135

[gRegorLove]

This appears to be configurable already based on the comment here: https://github.com/indieweb/indieauth-client-php/blob/main/src/IndieAuth/Client.php#L192 and mentioned in the readme (though it would probably be good to have a more accurate UA with the lib name).

I'm not seeing a separate separate way to make HTTP requests, unless you meant the indieweb\http_rels lib.