Support oneof based conditional value

[goccy]

This PR adds two features related to oneof.

1. Set the value for the oneof field

Previously, when assigning a value to a oneof field, it was only possible to use autobind to assign the preconstructed oneof value as is.

However, from now on, it will be possible to create a value for each oneof the fields with a grpc.federation.field option.

Evaluate messages only if expr is true.
Finally, evaluate by and assign it to field.

message UserSelection {
  option (grpc.federation.message) = {
    messages {
      name: "m"
      message: "M"
    }
  };
  oneof user {
    User user_a = 1 [
      (grpc.federation.field).oneof = {
        expr: "m.value == $.value"
        messages {
          name: "ua"
          message: "User"
          args: { name: "user_id", string: "a" }
        }
        by: "ua"
      }
    ];

    User user_b = 2 [
      (grpc.federation.field).oneof = {
        expr: "m.value != $.value"
        messages {
          name: "ub"
          message: "User"
          args: { name: "user_id", string: "b" }
        }
        by: "ub"
      }
    ];
  }
}

2. Retrieving the value of oneof

sel is a variable name for got message.
user is a oneof's name of the above proto.

Normally, we cannot directly reference the name of oneof, but if all of the oneof fields have the same type, we can retrieve the value of any field that is not nil.

message GetResponse {
  option (grpc.federation.message) = {
    messages: [
      {
        name: "sel"
        message: "UserSelection"
        args { name: "value", string: "foo" }
      }
    ]
  };
  User user = 1 [(grpc.federation.field).by = "sel.user"];
}

[github-actions]

Code Metrics Report

	main (f3f58fc)	#33 (e2731c5)	+/-
Coverage	65.9%	66.1%	+0.2%
Code to Test Ratio	1:0.1	1:0.1	-0.0
Test Execution Time	3m32s	3m8s	-24s

Details

  |                     | main (f3f58fc) | #33 (e2731c5) |  +/-  |
  |---------------------|----------------|---------------|-------|
+ | Coverage            |          65.9% |         66.1% | +0.2% |
  |   Files             |             47 |            47 |     0 |
  |   Lines             |           6974 |          7295 |  +321 |
+ |   Covered           |           4595 |          4819 |  +224 |
- | Code to Test Ratio  |          1:0.1 |         1:0.1 |  -0.0 |
  |   Code              |          52128 |         57951 | +5823 |
+ |   Test              |           6167 |          6411 |  +244 |
+ | Test Execution Time |          3m32s |          3m8s |  -24s |

Code coverage of files in pull request scope (72.0% → 71.9%)

Files	Coverage	+/-
generator/code_generator.go	86.9%	-0.1%
grpc/federation/federation.pb.go	22.8%	+0.4%
internal/testutil/builder.go	89.2%	+0.5%
internal/testutil/cmpopt.go	100.0%	0.0%
resolver/cel.go	69.2%	+2.6%
resolver/graph.go	87.2%	-8.7%
resolver/message.go	85.2%	+0.7%
resolver/oneof.go	76.9%	-3.1%
resolver/resolver.go	87.8%	-0.0%
resolver/types.go	90.5%	0.0%
resolver/value.go	76.5%	+0.1%
source/file.go	74.2%	-2.2%
source/location.go	54.0%	-10.3%

---

Reported by octocov