Feature Request: Add embedded replicas support to C API

Current Behavior

Currently, the C API (libsql_experimental.h) only supports remote database mode through the libsql_config structure:

typedef struct {
  const char *db_path;
  const char *primary_url;  // Only one URL parameter
  const char *auth_token;
  char read_your_writes;
  const char *encryption_key;
  int sync_interval;
  char with_webpki;
} libsql_config;

This structure doesn't support the embedded replicas functionality that is available in the Rust API through the Builder pattern:

```rust
// Rust API supports different modes:
Builder::new_remote_replica(path, url, auth_token)
Builder::new_synced_database(path, url, auth_token)

Problem
Libraries that use the C API (like op-sqlite) cannot implement embedded replicas functionality because:

• There's no way to distinguish between primary URL and sync URL
• No way to specify the desired mode (remote/embedded replica/synced database)
• All operations go directly to the remote database instead of using local replica

Start with libsql_experimental.h and the libsql_config structure, then compare its current options with the Rust Builder constructors named in the issue. Trace how the C API currently selects remote mode and how embedded replicas are represented in the Rust API. Done means C callers can distinguish primary and sync URLs and select the supported database modes.