feat(gax): add jitter knob to RetrySetting

[op]

Add a jitter knob to RetrySetting but keep it disabled by default.

[Copilot]

Pull request overview

This PR adds an optional “jitter” control to the gcloud-gax retry behavior, while keeping jitter disabled in all default retry settings across clients.

Changes:

• Extend foundation/gax::RetrySetting with a jitter: bool knob (defaulting to false).
• Apply tokio-retry2’s strategy::jitter to retry delays when the knob is enabled.
• Update service-specific default retry settings to explicitly set jitter: false.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
spanner/src/apiv1/spanner_client.rs	Adds jitter: false to the client’s default RetrySetting literal.
spanner/src/admin/mod.rs	Adds jitter: false to the admin default retry setting.
kms/src/grpc/apiv1/kms_client.rs	Adds jitter: false to the KMS default RetrySetting literal.
foundation/longrunning/src/autogen/operations_client.rs	Adds jitter: false to the longrunning operations default retry setting.
foundation/gax/src/retry.rs	Introduces RetrySetting.jitter and applies jitter mapping to retry strategies when enabled.
foundation/gax/Cargo.toml	Enables the tokio-retry2 jitter feature to support jittered backoff.
bigquery/src/grpc/apiv1/bigquery_client.rs	Adds jitter: false to the BigQuery default RetrySetting literal.
artifact-registry/src/grpc/apiv1/artifact_registry_client.rs	Adds jitter: false to the Artifact Registry default RetrySetting literal.

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