[OpAMP] Support `AcceptsOpAMPConnectionSettings`

**Describe the enhancement:**

Add support to Fleet Server for offering OpAMP connection settings to clients, allowing Fleet to redirect or reconfigure how a managed client connects back to an OpAMP server (endpoint, headers, TLS certificates, etc.). This corresponds to the `OffersConnectionSettings` server capability applied to the OpAMP channel itself.

**Capability advertisement vs. offering:**

Per the OpAMP spec, Fleet Server MUST advertise `OffersConnectionSettings` in the [`capabilities` field](https://opentelemetry.io/docs/specs/opamp/#servertoagentcapabilities) of the first `ServerToAgent` message unconditionally. However, Fleet Server may only actually send an OpAMP-channel `ConnectionSettings` offer to clients that have advertised [`AcceptsOpAMPConnectionSettings`](https://opentelemetry.io/docs/specs/opamp/#agenttoservercapabilities) in their most recent `AgentToServer` message. Advertisement is unconditional; offering is gated on client capability. See the [Connection Settings Management section](https://opentelemetry.io/docs/specs/opamp/#connection-settings-management) of the spec.

**Testing:**

This feature should be tested against an upstream OTel Collector (contrib distribution) managed by the OpAMP Supervisor. The supervisor does NOT advertise `AcceptsOpAMPConnectionSettings` by default — the supervisor config must set `capabilities.accepts_opamp_connection_settings: true`. See the [OpAMP Supervisor README](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/cmd/opampsupervisor) for instructions on running the supervisor, and the [OTel Collector contrib releases](https://github.com/open-telemetry/opentelemetry-collector-releases/releases) page for the collector binary.

**Describe a specific use case for the enhancement or feature:**

Rotating credentials or migrating managed clients between OpAMP endpoints without manual reconfiguration. For example, when a Fleet Server certificate rotates, Fleet can push new TLS material to all managed agents so they continue connecting without operator intervention.