Add TLS session resumption support on .NET 8.0+

[sylwiaszunejko]

Summary

This PR adds TLS session resumption (session ticket reuse) for connections to Scylla/Cassandra nodes when running on .NET 8.0 or later. Subsequent TLS connections to the same host reuse previously negotiated session parameters.

On older runtimes (netstandard2.0), behavior is unchanged.

Motivation

Each new TLS connection currently performs a full handshake. For clusters with many nodes or frequent connection churn, this adds latency. .NET 8.0 introduced SslClientAuthenticationOptions.AllowTlsResume, which enables the runtime to cache and reuse TLS session tickets — but only when the same SslClientAuthenticationOptions instance is reused across connections to the same host.

Changes

Project configuration

• Add net8.0 as a target framework alongside netstandard2.0 (multi-targeting)
• Update LangVersion to latest for modern C# features
• Suppress SYSLIB0051, SYSLIB0012, SYSLIB0039, CA1416 warnings for net8.0 builds
• Conditionally reference the Cassandra project with the correct target framework in tests

PlatformHelper

• Report .NET 8.0+ in GetTargetFramework() when built with NET8_0_OR_GREATER

SSLOptions

• Add EnableSessionResumption property (defaults to true)
• Add SetEnableSessionResumption(bool) fluent setter
• Add unit tests for the new property

TlsSessionTicketCache (new class)

• Per-cluster cache of SslClientAuthenticationOptions keyed by server name
• Returns the same instance for repeated connections to the same host, which is required by .NET for session ticket reuse
• Sets AllowTlsResume based on SSLOptions.EnableSessionResumption
• No-op on netstandard2.0
• Add unit tests covering caching behavior, AllowTlsResume, revocation mode, and callback propagation

Configuration

• Add TlsSessionTicketCache property, initialized when SSL is configured

TcpSocket / Connection

• Pass TlsSessionTicketCache from Configuration through Connection to TcpSocket
• On NET8_0_OR_GREATER, use AuthenticateAsClientAsync(SslClientAuthenticationOptions) with cached options
• Keep existing AuthenticateAsClientAsync(string, ...) path for netstandard2.0

Usage

TLS session resumption is enabled by default when using .NET 8.0+. No configuration changes are needed.

To disable it:

var sslOptions = new SSLOptions().SetEnableSessionResumption(false);
var cluster = Cluster.Builder()
    .AddContactPoint("node1.example.com")
    .WithSSL(sslOptions)
    .Build();

Fixes: https://scylladb.atlassian.net/browse/DRIVER-171