Add options to allow passing certificates by string

[bernhard-efler]

Since PHP 8.1 (URL) and curllib 7.71.0 it is possible to pass SSL Certificates as strings.

To support this two new request options are created.
I since it would be more convinent to change the current options, but that would require a breaking change.

This feature was requestes four years ago in #2036, but at the time curl was ned capable.

[GrahamCampbell]

Unfortunately still not enough time has passed for this to be viable, for the same reasons as previously. I think we should come back to this in another 4 years once we can be sure everyone has a new enough PHP and curl version. I don't really want some Guzzle functionality to fail at runtime because it's not supported (nor to implement a polyfill for the reasons discussed in the linked PR). Thanks anyway.