Add BCR GitHub workflow and configuration

[Copilot]

Changes

Implements automated publishing to the Bazel Central Registry following the pattern established in opentelemetry-proto.

BCR Configuration (.bcr/)

• metadata.template.json: Module metadata with current BCR maintainer (Keith Smiley) and recent release publisher (Marc Alff)
• presubmit.yml: Test matrix across debian10, macos, ubuntu2004, windows on Bazel 7.x, 8.x, 9.*
• source.template.json: Archive URL template with placeholder substitution
• README.md: Publishing workflow documentation, PAT setup, fork registry recommendations

Workflow

.github/workflows/publish-to-bcr.yml triggers on release publication or manual dispatch, using bazel-contrib/[email protected] to submit entries to the official registry.

Documentation

README.md updated with BCR section:

bazel_dep(name = "opentelemetry-cpp", version = "x.y.z")

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

Original prompt

With the same logic in open-telemetry/opentelemetry-proto@21697ec please provide BCR github workflow, config and README. See https://github.com/bazelbuild/bazel-central-registry/tree/main/modules/opentelemetry-cpp for reference configuration.
Concerning maintainers you can use the current in BCR and add publishers of the 10 previous tags of https://github.com/open-telemetry/opentelemetry-cpp as this is for the source project. You can recommend the use of a fork registry but the one actually configure must be the official https://github.com/bazelbuild/bazel-central-registry

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