cmake: add support for `CURL_DEFAULT_SSL_BACKEND`

cmake: add support for `CURL_DEFAULT_SSL_BACKEND`

Allow overriding the default TLS backend via a CMake setting.

E.g.:
`cmake [...] -DCURL_DEFAULT_SSL_BACKEND=mbedtls`

Accepted values: bearssl, gnutls, mbedtls, openssl, rustls,
schannel, secure-transport, wolfssl

The passed string is baked into the curl/libcurl binaries.
The value is case-insensitive.

We added a similar option to autotools in 2017 via
c7170e20d0a18ec8a514b4daa53bcdbb4dcb3a05.

TODO: Convert to lowercase to improve reproducibility.

Closes #11774