Add fluent Assertion builder for type-safe assertion definitions

[gonace]

Summary

Implements a fluent builder API for Cronitor assertions, providing type safety and discoverability when defining monitor assertions instead of using raw strings.

Key Changes

• New Assertion class in Cronitor.Constants with fluent builder pattern

  • Assertion.Metric provides builders for Duration, Count, and ErrorCount
  • Assertion.Response provides builders for Code, Time, Body, and parameterized Json() and Header() methods
  • Each builder supports comparison operators: Equals(), LessThan(), GreaterThan(), and Contains()

• New AssertionRule class to represent assertion expressions

  • Implements implicit conversions to/from string for backward compatibility
  • Custom JSON converter (AssertionRuleConverter) for proper serialization/deserialization
  • Decorated with [JsonConverter] attribute for automatic handling

• Updated Monitor.Assertions property to use IEnumerable<AssertionRule> instead of IEnumerable<string>

  • Maintains backward compatibility through implicit string conversion

• Updated tests and builders to use the new fluent API

  • Added comprehensive AssertionTests class with 11 test cases covering all assertion types and serialization
  • Updated MonitorTypeTests, JobBuilder, and request tests to use AssertionRule type

• Updated README with usage examples and marked the assertion language feature as complete

Notable Implementation Details

• The AssertionBuilder class uses a private constructor pattern to enforce fluent API usage
• Assertion strings are built dynamically with proper spacing and operator placement
• JSON serialization treats AssertionRule objects as simple strings for API compatibility
• The builder supports both simple assertions (e.g., metric.duration < 30s) and parameterized ones (e.g., response.json user.count > 10)