[NitinKumar004]

Adds CloudWatch Logs StartLiveTail.

• The provider keeps a registry of Live Tail sessions. PutLogEvents fans stored events out to them after the stream lock is released. The fan-out only appends to a per-session buffer and never blocks on a client.
• Each session sends a sessionUpdate every second, including empty ones. An update carries at most 500 events, and when more matched it is sampled and sessionMetadata.sampled is set. A slow client gets up to 10 queued updates or 5000 events, and after that the oldest are dropped.
• The request is validated per the API reference: 1 to 10 log group ARNs, no trailing *, logStreamNames and logStreamNamePrefixes mutually exclusive and only with one group, a 1024-character pattern parsed with the filterpattern package, and a limit of 15 concurrent sessions (LimitExceededException). An unknown group returns ResourceNotFoundException.
• A session ends with SessionTimeoutException after 3 hours (liveTailSessionTimeout), measured on the options clock. Data protection masking applies, since events are masked at ingest.
• Wire: the event stream is initial-response, then sessionStart, then sessionUpdate frames. Kinesis SubscribeToShard's frame writer moved to a shared server/wire/awseventstream package, and Kinesis now uses it too.
• Dispatch uses X-Amz-Target, so the stream-logs. host from current SDKs works, and so does streaming-logs. from older SDKs and the AWS CLI.
• Shutdown: a session ends when the client disconnects. serve also ends open streams when it shuts down, through a stream-shutdown base context (wire.StreamContext), so they no longer hold the graceful shutdown until its 10s timeout. statusWriter gets Unwrap so frames still flush with --log-requests.

Clients send StartLiveTail to a prefixed host: stream- from aws-sdk-go-v2 and streaming- from botocore. Against localhost, those names do not resolve. The usage note is in docs/coverage/nongoals/logging.md. It covers a wildcard-DNS host such as http://logs.localtest.me:4566 or an /etc/hosts entry for the CLI, inject_host_prefix=False for boto3, and smithyhttp.DisableEndpointHostPrefix in an API option for the Go SDK. Tests cover both prefixes and the Go SDK option.

Part of #1476