bearssl: support CURLOPT_CAINFO_BLOB

[AzazKamaz]

Just implement support to use CURLOPT_CAINFO_BLOB with bearssl.
Also fixed bearssl support building error on hostname = NULL; while having const char * const hostname = SSL_HOST_NAME();

There are four ways to implement support:

• have one function with parsing cert file from the buffer and two ways to get buffer. I don't want to read an entire file into memory so denied
• have two big functions with similar code buf from different sources. Too big code repeat so denied
• parse cert file iteratively while having state struct. Need to split function into some struct, init(), dispose() and update(). Denied because of complexity
• have some switches in the cert parsing function. The easiest way with a minimum amount of modification. Implemented this way

[AzazKamaz]

Actually, I am force pushing my branch to stay away from commits like "fix formatting" or "change initialization way"

[bagder]

The CURLOPT_CAINFO_BLOB.3 man page need an update to clarify that bearssl also supports this option from >= 7.79.0, right?

[AzazKamaz]

I have added info to docs and used DEBUGASSERT for check.
Also I have changed source->len ? source->len : strlen(source->data) to source->len because documentation requires pointer and size and strlen will cause problems if buffer is zero-length and not zero-determinated.
And added check source->len <= (size_t)INT_MAX same as in OpenSSL backend

[bagder]

Thanks!