feat: Add auth.tokenFile support for file-based token authentication

[xufeiranfree]

This pull request introduces the auth.tokenFile configuration option for both frpc.toml and frps.toml files, providing a token-based authentication.

Changes:

• Added the auth.tokenFile configuration option to the documentation.
• Modified the authentication logic to read the token from a file specified by auth.tokenFile if auth.token is not set.
• Included additional documentation in the README.md to guide users on how to utilize the new auth.tokenFile option.
• Implemented and added test cases to verify the functionality of the auth.tokenFile configuration.

Notes:

• This feature is optional and maintains backward compatibility with configurations that use auth.token.

Closes:
Closes #4353

[fatedier]

Please fully discuss and reach an agreement in the issue before submitting a PR; otherwise, unexpected PRs may waste everyone's time. This is not the solution I am looking for; I hope for a unified mechanism to configure and reference secrets.