[Backport] Add C# 9.0 record support to serializer (#7108)

[ReubenBond]

Backport for 3.5.0

Backport of #7108

[ReubenBond]

This PR is contingent on #7121, so that should be merged first

[jonathansant]

@ReubenBond Does this mean records have to have the Serializable attribute?

[ReubenBond]

The same rules apply for records as for other types: they don't need [Serializable] if they directly appear in another serializable type or a grain interface method signature. I'd recommend adding [Serializable] to any serializable type, though, since that is a more explicit way of telling the code generator to generate a serializer for a type.

[akourbat]

Does one still have to use Immutable<T> to tell the runtime not to deep-copy the object or for record types it is not necessary anymore (records are immutable by design)?

[ReubenBond]

Does one still have to use Immutable<T> to tell the runtime not to deep-copy the object or for record types it is not necessary anymore (records are immutable by design)?

Records aren't necessarily immutable. They can have mutable fields or fields of mutable types (dictionary, list, etc).

Note that you can annotate a type with [Immutable] instead of wrapping it with Immutable<T> if you prefer