Move protos to sdk-core-protos, add symlink in root

Nix sandboxes builds such that they cannot escape their manifest
directory. By moving the `protos` into `sdk-core-protos` and adding a
symlink in the root to this new location, Nix will be able to build
crates that depend on `sdk-core-protos`.

Otherwise, the build script will fail:

    Error: Custom { kind: Other, error: "protoc failed: Could not make proto path relative: ../protos/local/temporal/sdk/core/core_interface.proto: No such file or directory\n" }

README.md

@@ -86,7 +86,7 @@ equivalent.
 
 ## Proto files
 
-This repo uses a subtree for upstream protobuf files. The path `protos/api_upstream` is a
+This repo uses a subtree for upstream protobuf files. The path `sdk-core-protos/protos/api_upstream` is a
 subtree. To update it, use:
 
 `git pull --squash -s subtree ssh://[email protected]/temporalio/api.git master --allow-unrelated-histories`
@@ -100,9 +100,9 @@ subdirectory of that repo, we just copy the files with read-tree:
 # add sdk-java as a remote if you have not already
 git remote add -f -t master --no-tags testsrv-protos [email protected]:temporalio/sdk-java.git
 # delete existing protos
-git rm -rf protos/testsrv_upstream
+git rm -rf sdk-core-protos/protos/testsrv_upstream
 # pull from upstream & commit
-git read-tree --prefix protos/testsrv_upstream -u testsrv-protos/master:temporal-test-server/src/main/proto
+git read-tree --prefix sdk-core-protos/protos/testsrv_upstream -u testsrv-protos/master:temporal-test-server/src/main/proto
 git commit
 ```
 

protos

@@ -0,0 +1 @@
+sdk-core-protos/protos/

sdk-core-protos/build.rs

@@ -4,7 +4,7 @@ static ALWAYS_SERDE: &str = "#[cfg_attr(not(feature = \"serde_serialize\"), \
                                derive(::serde::Serialize, ::serde::Deserialize))]";
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    println!("cargo:rerun-if-changed=../protos");
+    println!("cargo:rerun-if-changed=./protos");
     let out = PathBuf::from(env::var("OUT_DIR").unwrap());
     let descriptor_file = out.join("descriptors.bin");
     tonic_build::configure()
@@ -115,17 +115,17 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .file_descriptor_set_path(#[allow(clippy::needless_borrow)] &descriptor_file)
         .compile(
             &[
-                "../protos/local/temporal/sdk/core/core_interface.proto",
-                "../protos/api_upstream/temporal/api/workflowservice/v1/service.proto",
-                "../protos/api_upstream/temporal/api/operatorservice/v1/service.proto",
-                "../protos/testsrv_upstream/temporal/api/testservice/v1/service.proto",
-                "../protos/grpc/health/v1/health.proto",
+                "./protos/local/temporal/sdk/core/core_interface.proto",
+                "./protos/api_upstream/temporal/api/workflowservice/v1/service.proto",
+                "./protos/api_upstream/temporal/api/operatorservice/v1/service.proto",
+                "./protos/testsrv_upstream/temporal/api/testservice/v1/service.proto",
+                "./protos/grpc/health/v1/health.proto",
             ],
             &[
-                "../protos/api_upstream",
-                "../protos/local",
-                "../protos/testsrv_upstream",
-                "../protos/grpc",
+                "./protos/api_upstream",
+                "./protos/local",
+                "./protos/testsrv_upstream",
+                "./protos/grpc",
             ],
         )?;