Improve ssh host-key verification and timeout handling.

[frankgiordano]

PR updates for UssCmd.java:

• Enabled strict SSH host-key verification by default using the user's default known_hosts file.
• Added support for the existing zowe.sdk.allow.insecure.connection system property as an explicit opt-out of host-key verification.
• Added a warning log when insecure SSH host-key verification is enabled.
• Only loads the known_hosts file when secure host-key verification is enabled.
• Added timeout handling to the SSH session connection.
• Added timeout handling to the SSH channel connection.
• Added a timeout for remote USS command execution to protect against network hangs or stuck processes.
• Validated that the command timeout is greater than zero.
• Used System.nanoTime() for reliable elapsed-time measurement and converted the configured timeout from milliseconds to nanoseconds.
• Updated Javadocs to document SSH session, channel, and command execution timeout behavior.
• Ensured SSH sessions and channels are properly disconnected through AutoCloseable wrappers.
• Security
• SSH host keys are now verified against the user's known_hosts file by default. Unknown or changed host keys are rejected unless the user explicitly enables the existing insecure connection property.
• The insecure option logs a warning and disables host-key verification using StrictHostKeyChecking=no.