✨ Add support for static client certificates defined in fastly.toml.

[acw]

As it says in the CHANGELOG entry:

Add support for supplying client certificates in fastly.toml, through the use of the client_cert_info table, which must have one of a "certificate" or "certificate_file" key, as well as one of a "key" and "key_file" key. The "_file" variants can be used to point to certificate/key files on disk, whereas the non-"_file" variants should be multi-line string constants in the toml. In all cases, they should be in PEM format.