Fixes rancher#221

Add support for OpenSSH private key format for Git SSH authentication.

* Update `createAuthFromOpts` function in `internal/cmd/cli/gitcloner/cloner.go` to handle OpenSSH private keys.
* Add logic to read and parse OpenSSH private keys in `createAuthFromOpts` function.
* Add support for known hosts file handling in `createAuthFromOpts` function.
* Add fallback to insecure host key callback if known hosts file is not provided.
* Add logic to handle username and password authentication in `createAuthFromOpts` function.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/rancher/fleet/issues/221?shareId=XXXX-XXXX-XXXX-XXXX).

* Update `GitCloner` struct to include `OAuthToken` field
* Update `NewCmd` function to support OAuth token authentication
* Add `TestArgsAreSetWithOAuthToken` test case to verify OAuth token handling