[protobuf] Represent TypeSpec optional properties as optional fields

[witemple-msft]

Resolves a long-outstanding capability issue with the protobuf emitter not supporting optional because it wasn't specified in the proto3 language specification when this emitter was created.

There is a fair amount of nuance about how "optionality" is represented in protobuf (everything is actually optional in a protobuf message -- the only difference is whether "presence" is explicit or implicit, so optional is really rather a misnomer).

• For Protobuf messages, we don't emit optional because Protobuf message-typed fields always have explicit presence discipline. Applications are responsible for validating whether an "unset" field is acceptable.
• For Protobuf scalars, we emit optional. However, the alternative is not that the message will fail to validate if the field is not provided, rather that the client will provide the default value of that scalar (zero, empty string, etc.).
• For repeated/map-typed fields (TypeSpec arrays, Protobuf.Map), we produce a warning. Due to the nature of how repeated fields are encoded (and map desugars to a repeated key-value pair), it is impossible to differentiate empty from unset in an encoded protobuf message.

Closes #10499
Closes #4311

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/protobuf@10598

commit: 52848d1

[github-actions]

All changed packages have been documented.

• ✅ @typespec/protobuf

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@typespec/protobuf - feature ✏️

Map TypeSpec optionality (?) to protobuf optional where appropriate.,> ,> - optional is applied to fields with protobuf scalar types to set explicit presence.,> - optional is not applied to fields with message types, because they always have explicit presence.,> - Attempting to convert a TypeSpec optional property where the type is an array or Protobuf.Map instance produces a warning, because protobuf cannot differentiate between "empty" and "unset" repeated/map-typed fields.

[azure-sdk]

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