attestation: add awsSEVSNP as new variant (#1900)

* variant: move into internal/attestation
* attesation: move aws attesation into subfolder nitrotpm
* config: add aws-sev-snp variant
* cli: add tf option to enable AWS SNP

For now the implementations in aws/nitrotpm and aws/snp
are identical. They both contain the aws/nitrotpm impl.
A separate commit will add the actual attestation logic.

internal/api/attestationconfigapi/BUILD.bazel

@@ -14,11 +14,11 @@ go_library(
     deps = [
         "//internal/api/client",
         "//internal/api/fetcher",
+        "//internal/attestation/variant",
         "//internal/constants",
         "//internal/logger",
         "//internal/sigstore",
         "//internal/staticupload",
-        "//internal/variant",
     ],
 )