Add ca_cert credential for http connection

We connect to a Managed ClickHouse cluster whose server certificate is signed by an internal/Yandex Root CA.

`clickhouse-connect` supports a file-path argument ca_cert that lets a client trust such a certificate chain. The dbt-clickhouse credential schema, however, exposes only the boolean flag verify and it is not possible to add ca_cert .pem file like in clickhouse_connect.

Proposal: add an optional string field ca_cert to class ChHttpClient._create_client:
```
return clickhouse_connect.get_client(
...,
verify=credentials.verify,
ca_cert=credentials.ca_cert
)
```

Start at ChHttpClient._create_client and the dbt-clickhouse credential schema, then trace how the existing verify field reaches clickhouse_connect.get_client. Add the optional ca_cert credential and pass it through alongside verify. Done means a configured PEM path is accepted and forwarded without changing behavior when it is omitted.

Written by the indexing model from the issue text.