Support CA cert bundles

[letmaik]

Closes #2221.

[ghost]

letmaik/ca-cert-bundles@19636 aka 20210301.5 vs main ewma over 20 builds from 19331 to 19632

[eddyashton]

Since a bundle is "just" a concatenation, I wonder if this should be just the plural set_ca_certs rather than set_ca_certs_bundle? If I have a single cert I want to add, it may not be clear to me how I combine/wrap those into a bundle? But if I saw set_ca_certs I'd probably blindly submit my single entry there? And in the reverse case where I already have a bundle (and know its a mere concatenation), I'd still be happy with set_ca_certs? This bike shed has a very nice felt roof.

[letmaik]

I think semantically there's a big difference and I would rather be more explicit at the possible cost of confusing people so that they have to check the docs.