feat: output checksums to artifacts info (#3548)

Following #3540, output artifacts' checksums to the artifact info.
This addition makes it easier to consume the checksums, especially when
running from e.g. GitHub Actions.

New tests:
1. Add a check for the checksum in the extra field. 
2. Add a test for every checksum algorithm (to see that it doesn't break
for any algo's output).
3. Add a case of a binary and an extra file (to see that the logic
doesn't break when there's a mix).

p.s.
While working on that, I noticed that the convention for extra fields is
actually to use UpperCamelCase rather than lower.
I was mistaken because I looked at the subfields of the "DockerConfig"
extra field.
I think it's a good idea to fix it quickly, before the next release
rolls and it becomes a compatibility issue.
I took the liberty to fix it here as an extra commit. Please let me know
if you want it to be in a separate PR.

---
Tests:
```
go test
  • refreshing checksums                             file=binary_bar_checksums.txt
  • refreshing checksums                             file=binary_bar_checksums.txt
  • refreshing checksums                             file=binary_bar_checksums.txt
PASS
ok  	github.com/goreleaser/goreleaser/internal/pipe/checksums	0.184s
```

Co-authored-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

internal/pipe/checksums/checksums.go

@@ -19,6 +19,10 @@ import (
 	"github.com/goreleaser/goreleaser/pkg/context"
 )
 
+const (
+	artifactChecksumExtra = "Checksum"
+)
+
 var (
 	errNoArtifacts = errors.New("there are no artifacts to sign")
 	lock           sync.Mutex
@@ -137,11 +141,17 @@ func refresh(ctx *context.Context, filepath string) error {
 	return err
 }
 
-func checksums(algorithm string, artifact *artifact.Artifact) (string, error) {
-	log.WithField("file", artifact.Name).Debug("checksumming")
-	sha, err := artifact.Checksum(algorithm)
+func checksums(algorithm string, a *artifact.Artifact) (string, error) {
+	log.WithField("file", a.Name).Debug("checksumming")
+	sha, err := a.Checksum(algorithm)
 	if err != nil {
 		return "", err
 	}
-	return fmt.Sprintf("%v  %v\n", sha, artifact.Name), nil
+
+	if a.Extra == nil {
+		a.Extra = make(artifact.Extras)
+	}
+	a.Extra[artifactChecksumExtra] = fmt.Sprintf("%s:%s", algorithm, sha)
+
+	return fmt.Sprintf("%v  %v\n", sha, a.Name), nil
 }

internal/pipe/checksums/checksums_test.go

@@ -344,6 +344,19 @@ func TestPipeCheckSumsWithExtraFiles(t *testing.T) {
 					require.Contains(t, string(bts), "3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855  "+want)
 				}
 			}
+
+			_ = ctx.Artifacts.Visit(func(a *artifact.Artifact) error {
+				if a.Path != file {
+					return nil
+				}
+				if len(tt.ids) > 0 {
+					return nil
+				}
+				checkSum, err := artifact.Extra[string](*a, artifactChecksumExtra)
+				require.Nil(t, err)
+				require.NotEmptyf(t, checkSum, "failed: %v", a.Path)
+				return nil
+			})
 		})
 	}
 }

internal/pipe/docker/docker.go

@@ -20,7 +20,7 @@ import (
 
 const (
 	dockerConfigExtra = "DockerConfig"
-	dockerDigestExtra = "digest"
+	dockerDigestExtra = "Digest"
 
 	useBuildx = "buildx"
 	useDocker = "docker"