Rekhoff/csharp default field values

[rekhoff]

Description of Changes

This is the implementation of issue #3191. This adds a Default attribute to C# module fields.

Note: In C#, attribute arguments must be compile-time constants, which means you can't directly use non-constant expressions like new expressions, method calls, or dynamic values in attribute constructors.
(Ref: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/attributes#2324-attribute-parameter-types)

For this reason, these default values are limited to primitive types, enums, and strings.
This includes (shown as C# Type (BSATN type):

• bool (Bool)
• sbyte (I8)
• byte (U8)
• short (I16)
• ushort (U16)
• int (I32)
• unit (U32)
• long (I64)
• ulong (U64)
• float (F32)
• double (F64)
• enum (Enum)
• string (String)
• null (RefOption) <- Nullable type

Because of C# limitations, for nullable and complex data types, such as a struct, can take use [Default(null)] to populate values with null defaults. This allows things like structs to workaround the non-constant expressions in attribute constructors limitation by allowing these complex types to still be able to be added as new column tables.

The int type can also be in the form of Hex or Binary literals, such as [Default(0x2A)] or [Default(0b00101010)]

Both Decimal (like [Default(3.14m)]) and Char (like [Default('A')]) are unsupported types in BSATN and will still return BSATN0001 errors.

API and ABI breaking changes

Not API breaking.
This change only adds the [Default(value)] attribute logic.
Using the [Default(value)] attribute with older versions SpacetimeDB C# modules will result in an error.

Expected complexity level and risk

2

Testing

Local testing of this requires use of CLI changes in #3278

• Regression test of functionality added.

[github-actions]

Some missing or dangling .meta found. Fix commits are needed.

Status	File
Not committed	global.json.meta

[github-actions]

Some missing or dangling .meta found. Fix commits are needed.

Status	File
Not committed	global.json.meta

[jdetter]

I have not manually tested this but the regression tests are easy to understand and are passing. Thanks Ryan this looks great 👍