feat(19.3-03): summer.yaml build.tags and the summercms_s3 tag that links the S3 uploads driver

- Manifest.Tags from build.tags (^[a-z0-9_]+$, duplicates dropped), round-tripped by SaveManifest
- summer build passes -tags=<list> through goBuildArgs and prints the build tags line
- attach: s3blob imported only by s3_on.go (//go:build summercms_s3); an untagged binary
  refuses an s3:// bucket_url naming the tag, build.tags and -tags
- go mod tidy adds the AWS SDK v2 modules as indirect requirements
- storage, modules, scaffolding, configuration and scaling docs plus the lagoon README

go.sum

@@ -153,6 +153,12 @@ github.com/golang/protobuf v1.5.4 h1:i7eJL8qZTpSEXOPTxNKhASYpMn+8e5Q6AdndVa1dWek
 github.com/golang/protobuf v1.5.4/go.mod h1:lnTiLA8Wa4RWRcIUkrtSVa5nRhsEGBg48fD6rSs7xps=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/google/go-replayers/grpcreplay v1.3.0 h1:1Keyy0m1sIpqstQmgz307zhiJ1pV4uIlFds5weTmxbo=
+github.com/google/go-replayers/grpcreplay v1.3.0/go.mod h1:v6NgKtkijC0d3e3RW8il6Sy5sqRVUwoQa4mHOGEy8DI=
+github.com/google/go-replayers/httpreplay v1.2.0 h1:VM1wEyyjaoU53BwrOnaf9VhAyQQEEioJvFYxYcLRKzk=
+github.com/google/go-replayers/httpreplay v1.2.0/go.mod h1:WahEFFZZ7a1P4VM1qEeHy+tME4bwyqPcwWbNlUI1Mcg=
+github.com/google/martian/v3 v3.3.3 h1:DIhPTQrbPkgs2yJYdXU/eNACCG5DVQjySNRNlflZ9Fc=
+github.com/google/martian/v3 v3.3.3/go.mod h1:iEPrYcgCF7jA9OtScMFQyAlZZ4YXTKEtJ1E6RWzmBA0=
 github.com/google/s2a-go v0.1.9 h1:LGD7gtMgezd8a/Xak7mEWL0PjoTQFvpRudN895yqKW0=
 github.com/google/s2a-go v0.1.9/go.mod h1:YA0Ei2ZQL3acow2O62kdp9UlnvMmU7kA6Eutn0dXayM=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=