Add Host Bindings feature

[schlessera]

Pull Request Type

This is a:

• Bug fix
• New feature
• Documentation improvement
• Code quality improvement

Context

This PR adds support for binding specific hostnames to specific IP addresses, bypassing DNS resolution.

This is useful for:

• Testing against specific server instances
• Load balancing and failover scenarios
• Connecting to servers via specific IPs while preserving the original hostname in the Host header
• Development environments where DNS may not be configured
• Preventing rebinding attacks

Detailed Description

New HostBindings utility class

A value object that validates and stores hostname-to-IP mappings. Key features:

• Security by default: IP addresses are validated using filter_var(FILTER_VALIDATE_IP) to prevent hostname injection attacks. Accepts both IPv4 and IPv6 addresses (including private/localhost ranges).
• Opt-out mechanism: For pre-validated inputs or non-standard formats, validation can be skipped via HostBindings::SKIP_IP_VALIDATION (use with caution).
• Normalization: Whitespace is trimmed from IP addresses.

Methods:

• has_host($host) - Check if a host has a mapping
• get_first_ip_for_host($host) - Get the first IP for a host (throws UnknownHost or MissingIpAddress if not available)
• get_all_ips_for_host($host) - Get all IPs for a host (throws UnknownHost if not found)

New exceptions

• UnknownHost - Thrown when requesting a host that isn't in the bindings
• MissingIpAddress - Thrown when a host has no IP addresses configured

New HOST_BINDINGS capability

Added to the Capability interface to allow transport capability testing.

Transport integration

Curl transport (Transport\Curl):

• Uses CURLOPT_CONNECT_TO (cURL 7.49.0+) as the preferred method
• Falls back to CURLOPT_RESOLVE (cURL 7.21.3+) for older cURL versions
• Falls back to URL rewriting for HTTP (not HTTPS) on very old cURL versions
• Properly handles IPv6 addresses by wrapping them in square brackets
• Preserves the original hostname in the Host header

Fsockopen transport (Transport\Fsockopen):

• Replaces the connection host with the mapped IP address
• Original hostname is preserved in the Host header

Usage

The host_bindings option accepts either an array (with automatic IP validation) or a pre-constructed HostBindings object:

// Using an array (IPs are validated)
$response = Requests::get('https://example.com/api', [], [
    'host_bindings' => [
        'example.com' => ['93.184.216.34', '2606:2800:220:1:248:1893:25c8:1946'],
    ],
]);

// Using a HostBindings object (for advanced control)
$bindings = new HostBindings(
    ['example.com' => ['custom-value']],
    HostBindings::SKIP_IP_VALIDATION
);
$response = Requests::get('https://example.com/api', [], [
    'host_bindings' => $bindings,
]);

Includes

• Comprehensive unit tests for HostBindings class (constructor validation, all methods)
• Unit tests for new exceptions
• Integration tests in Transport\BaseTestCase covering both transports
• Documentation in Requests::request() docblock