Add enable_stable_ips to PublicConnectivityRule

[khisakuni]

What was changed

Adds the enable_stable_ips field to the PublicConnectivityRule message.

Why?

Enables the stable IPs feature (still gated behind feature flag).

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

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Note

Medium Risk
Introduces a new public API field and bumps the advertised API version, which may require client regeneration and could affect consumers that validate schema/version headers.

Overview
Adds enable_stable_ips to PublicConnectivityRule in the connectivity rule proto (gated by temporal:versioning:min_version=v0.15.0) to allow opting into stable public egress IPs.

Bumps the Cloud Ops API version from v0.14.0 to v0.15.0 in README.md and VERSION to reflect the new schema.

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