Blazor forms model binding

[SteveSandersonMS]

In scope:

• Conversion (deserialization) of incoming form post data into .NET objects.
• Any layering work needed if there's an immediate plan to share code with Minimal

Out of scope:

• Any other aspect of the form handling system besides the pure data mapping part.

The following items were originally taken from #46688:

• Declarative binding
• Primitive values
• Collections
• Dictionaries
• Complex types
• Recursive complex types
• Error handling
• Security
• Extensibility
  • Globalization
  • Ignored properties.
  • Changing the property name.
  • Required properties.
  • Custom converters for a given property.
• Converter source generation
• Optimization
  • Removing allocations during prefix computation.
  • Removing allocations from reading the form.
  • Optimized form data structure for common form sizes
    • Evaluate using a list instead of a dictionary for small sets of keys. AdaptiveCapacityDictionary
    • Filtering out keys we don't care about when reading the dictionary.
    • Sizing the dictionary ahead of time to avoid resizing it.
    • Inlining implementation for primitive types (bypass the converter)
    • Avoid allocating a prefix collection when there is only one dictionary on the type hierarchy (since will only process a single set of keys).

However some of the above should probably really be out of scope for .NET 8, e.g., "converter source generation".

[javiercn]

Comparison against the System.Text.Json feature set:

• X AllowTrailingCommas
• V Converters
• V DefaultBufferSize
• X DefaultIgnoreCondition
• X DictionaryKeyPolicy: Gets or sets the policy used to convert a IDictionary key's name to another format, such as camel-casing.
• X Encoder: Gets or sets the encoder to use when escaping strings, or null to use the default encoder.
• X IgnoreNullValues
• X IgnoreReadOnlyFields
• X IgnoreReadOnlyProperties
• X IncludeFields Gets or sets a value that indicates whether fields are handled during serialization and deserialization. The default value is false.
• V MaxDepth: Gets or sets the maximum depth allowed when serializing or deserializing JSON, with the default value of 0 indicating a maximum depth of 64.
• X NumberHandling
• X PropertyNameCaseInsensitive: Gets or sets a value that indicates whether a property's name uses a case-insensitive comparison during deserialization. The default value is false.
• X PropertyNamingPolicy: Gets or sets a value that specifies the policy used to convert a property's name on an object to another format, such as camel-casing, or null to leave property names unchanged.
• X ReadCommentHandling:
• X ReferenceHandler: Gets or sets an object that specifies how object references are handled when reading and writing JSON.
• V TypeInfoResolver: If we decide to do SG.
• X UnknownTypeHandling: We don't deserialize anything we don't know about.

V are the things we potentially need,
X are the things we don't.