[codegen] Add fluss-codegen module for runtime code generation

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

Runtime code generation is essential for achieving high performance in data processing systems. Instead of using reflection or generic implementations, generated code can:

1. **Eliminate virtual method dispatch** - Direct field access and type-specific operations
2. **Enable JIT optimization** - Generated code can be better optimized by the JVM
3. **Reduce boxing/unboxing overhead** - Type-specific code avoids primitive boxing
4. **Support complex type comparisons** - Nested types (arrays, maps, rows) require specialized comparison logic

The initial use case is `RecordEqualiser` for comparing `InternalRow` instances, which is needed for:
- Change data capture (CDC) deduplication
- Aggregation state management
- Primary key table updates

### Solution

### Core Framework

| Component | Description |
|-----------|-------------|
| `CodeGeneratorContext` | Manages reusable code fragments, member variables, and class-level declarations |
| `JavaCodeBuilder` | Type-safe builder for constructing Java source code with fluent API |
| `CompileUtils` | Compiles generated source code using Janino with LRU caching |
| `GeneratedClass` | Wrapper holding generated source code and compiled class |
| `CodeGenException` | Exception type for code generation failures |

### Type-Safe API

- `Modifier` enum: PUBLIC, PRIVATE, PROTECTED, STATIC, FINAL, ABSTRACT, SYNCHRONIZED, VOLATILE, TRANSIENT
- `PrimitiveType` enum: BOOLEAN, BYTE, CHAR, SHORT, INT, LONG, FLOAT, DOUBLE, VOID
- `Param` class: Type-safe method parameter representation
- Helper methods: `mods()`, `params()`, `typeOf()`, `arrayOf()`

### Code Generators

| Generator | Output Interface | Description |
|-----------|------------------|-------------|
| `EqualiserCodeGenerator` | `RecordEqualiser` | Generates code for comparing two `InternalRow` instances |

### Supported Data Types

The `EqualiserCodeGenerator` supports all Fluss data types:

- **Primitive types**: BOOLEAN, TINYINT, SMALLINT, INT, BIGINT, FLOAT, DOUBLE
- **String types**: CHAR, VARCHAR, STRING
- **Binary types**: BINARY, VARBINARY, BYTES
- **Temporal types**: DATE, TIME, TIMESTAMP, TIMESTAMP_LTZ
- **Numeric types**: DECIMAL (with precision/scale)
- **Complex types**: ARRAY, MAP, ROW (nested)

### Features

- Field projection support for partial row comparison
- Compiled class caching with configurable cache size
- Janino dependency shaded to `org.apache.fluss.shaded.org.codehaus.janino` to avoid classpath conflicts
- Comprehensive Javadoc and package-info documentation

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start by reviewing the proposed CodeGeneratorContext, JavaCodeBuilder, CompileUtils, GeneratedClass, and EqualiserCodeGenerator components and their RecordEqualiser use case. Done means the fluss-codegen module provides the listed runtime-generation framework, supports the stated data types and field projection, includes Janino caching and shading, and has the described documentation.

Written by the indexing model from the issue text.