Amazon S3 backup support

Uses amazonka-s3, which auto discovers credentials.

Did some manual testing:

```
glean --tier test --db-root ~/glean backup myrepo/0 s3:mybucket/jade-test
glean --tier test --db-root ~/glean2 restore s3:mybucket/jade-test/myrepo.0
```

Configuration ~/.config/glean/server/test, though I think this doesn't
actually matter?

```
{
    "backup": {
        "allowed": ["myrepo"],
        "location": "s3:mybucket/jade-test"
    },
    "restore": {
        "enabled": true
    }
}
```

This feature really needs some documentation (especially given I don't
know if fb expected anyone external to immediately go and use it so it's
probably in a wiki somewhere?); I would appreciate the docs that exist
getting released, or I can maybe write some.

Author: lf-

glean.cabal.in

@@ -93,6 +93,10 @@ flag opt
 flag benchmarks
      default: False
 
+-- Support for backups to Amazon S3
+flag s3-support
+     default: True
+
 -- run tests that require clang
 flag clang-tests
      default: True
@@ -666,6 +670,30 @@ library db
         glean:stubs,
         glean:rocksdb,
 
+library db-backup-s3
+    import: fb-haskell, deps
+    visibility: private
+    hs-source-dirs: glean/db/backup/s3
+
+    exposed-modules: Glean.Database.Backup.S3
+    build-depends:
+        glean:util,
+        glean:if-internal-hs,
+        glean:if-glean-hs,
+        glean:db,
+        amazonka,
+        amazonka-s3,
+        conduit,
+        unliftio,
+        microlens,
+        http-client,
+
+    if flag(s3-support)
+        buildable: True
+    else
+        buildable: False
+
+
 -- Backend API, and a few things built on top
 library backend-api
     import: fb-haskell, fb-cpp, deps
@@ -712,6 +740,7 @@ library backend-local
         Glean.Backend.Local
         Glean.Backend.Logging
         Glean.Dump
+
     build-depends:
         glean:angle,
         glean:backend-api,
@@ -1138,6 +1167,10 @@ executable glean-server
         haskeline >=0.7.3 && <0.9,
         json
 
+    if flag(s3-support)
+        cpp-options: -DENABLE_S3=1
+        build-depends: glean:db-backup-s3
+
 library shell-lib
     import: fb-haskell, fb-cpp, deps
     hs-source-dirs: glean/shell
@@ -1266,6 +1299,10 @@ executable glean
         split,
         Glob,
 
+    if flag(s3-support)
+        cpp-options: -DENABLE_S3=1
+        build-depends: glean:db-backup-s3
+
 executable glean-hyperlink
     import: fb-haskell, fb-cpp, deps, exe
     hs-source-dirs: glean/demo
@@ -2160,6 +2197,27 @@ test-suite backup
         glean:stubs,
         glean:util
 
+test-suite backup-s3
+    import: test
+    type: exitcode-stdio-1.0
+    main-is: BackupTestS3.hs
+    ghc-options: -main-is BackupTestS3
+    build-depends:
+        glean:config,
+        glean:core,
+        glean:db,
+        glean:if-glean-hs,
+        glean:if-internal-hs,
+        glean:schema,
+        glean:stubs,
+        glean:util,
+        glean:db-backup-s3
+
+    if flag(s3-support)
+        buildable: True
+    else
+        buildable: False
+
 test-suite catalog
     import: test
     type: exitcode-stdio-1.0