feat: add custom TLS config support

[RequiemP]

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Description

This PR adds support for custom TLS configuration in Qdrant client. This enhancement allows users to provide their own ClientTlsConfig, offering greater flexibility for connecting to Qdrant in various environments

Changes:

1. Added a tls_config: Option<ClientTlsConfig> field to the QdrantConfig struct.
2. Added a tls_config: Option<ClientTlsConfig> field to the ChannelPool struct.
3. Introduced tls_config and set_tls_config methods to QdrantConfig for configuring TLS.
4. Updated make_channel method within ChannelPool to use the custom tls_config if provided, falling back to the default ClientTlsConfig::new().with_native_roots() otherwise.

Example Usage

let ca_cert_pem = std::fs::read_to_string("path/to/ca.crt")?;
let tls_config = ClientTlsConfig::new()
    .ca_certificate(Certificate::from_pem(ca_cert_pem));

let client = Qdrant::from_url("http://localhost:6334")
    .tls_config(Some(custom_tls_config))
    .build()?;