feat: add NewServerFromConfig function for embedded server config processing

[yixianOu]

Add NewServerFromConfig function for embedded server config processing

Overview

Resolves #7092

This PR introduces a new NewServerFromConfig() function that automatically processes configuration files for embedded NATS servers.

Problem

When using NATS embedded in Go applications, developers (such as me) often pass the ConfigFile field in the Options struct, but NewServer(opts *Options) doesn't automatically process this configuration file. This leads to confusion and requires additional manual steps to load the configuration.

Solution

Approach Comparison

In pr #7333, a solution was proposed to directly modify NewServer():

func NewServer(opts *Options) (*Server, error) {
    if opts.ConfigFile != _EMPTY_ && opts.configDigest == "" {
        if err := opts.ProcessConfigFile(opts.ConfigFile); err != nil {
            return nil, err
        }
    }
    // ...original code
}

However, directly modifying NewServer() might raise concerns about backward compatibility. Therefore, this PR introduces a dedicated NewServerFromConfig() function specifically to handle the ConfigFile field of *Options.

Implementation

This PR adds:

func NewServerFromConfig(opts *Options) (*Server, error) {
    if opts.ConfigFile != _EMPTY_ && opts.configDigest == "" {
        if err := opts.ProcessConfigFile(opts.ConfigFile); err != nil {
            return nil, err
        }
    }
    return NewServer(opts)
}

Testing

Added comprehensive test coverage:

• TestNewServerFromConfigFunctionality: Tests basic functionality and error handling scenarios
• TestNewServerFromConfigVsLoadConfig: Validates equivalence with traditional LoadConfig() approach using deep equality checks

Acknowledgments

Thanks to @ripienaar for the suggestion to create a separate function instead of modifying NewServer() directly.

Signing Off

Signed-off-by: orician lzjzxOYX201905@gmail.com

[neilalexander]

It's an odd API shape here to take both an *Options and expect a filename, do we want the function instead to just take a filename string or is there a specific reason for this?

[yixianOu]

It's an odd API shape here to take both an *Options and expect a filename, do we want the function instead to just take a filename string or is there a specific reason for this?

Because I want the NATS server, when embedded in a Go program, to accept both configuration file input and programmatic input via the *Options struct.

[neilalexander]

LGTM