feat: enable cdc support via header

[tyler-french]

Now anyone can try out chunking using Buildbuddy to test:

1. Sign up for a trial/free account at https://www.buildbuddy.io/
2. Get a token with write access
3. Use Bazel Fork from Support remote cache CDC bazelbuild/bazel#28437
4. Build!

USE_BAZEL_VERSION="tyler-french/9.1.0-cdc" bazel build //... \
  --disk_cache= \
  --experimental_remote_cache_chunking \
  --remote_header=x-buildbuddy-cdc-enabled=true \
  --check_direct_dependencies=off \
  --remote_cache=grpcs://remote.buildbuddy.io

[Copilot]

Pull request overview

Adds an opt-in mechanism for content-defined chunking (CDC) based on a gRPC request header, enabling clients (and the cache proxy) to turn on chunking without relying solely on experiment flags.

Changes:

• Introduce x-buildbuddy-cdc-enabled header support via chunking.EnabledViaHeader() and make it override the existing chunking experiment flag.
• Propagate the CDC header through the cache proxy and use it to enable proxy-side chunking for large writes.
• Add unit tests and Bazel BUILD deps for the new gRPC metadata usage.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
server/remote_cache/chunking/chunking.go	Adds header constant + header-based enablement logic and integrates it into Enabled().
server/remote_cache/chunking/chunking_test.go	Adds tests covering header enablement and precedence vs experiment flag.
server/remote_cache/chunking/BUILD	Adds gRPC metadata dependency for library and tests.
enterprise/server/util/proxy_util/proxy_util.go	Adds CDC header to the list of headers propagated by the cache proxy.
enterprise/server/util/proxy_util/BUILD	Adds dependency on the chunking package for header propagation.
enterprise/server/byte_stream_server_proxy/byte_stream_server_proxy.go	Enables proxy-side chunking based on the CDC header (and adjusts nil cache checks).

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[maggie-lou]

Nice!