Make OAuth Redirect URI Configurable

[saucow]

What I did

Made the OAuth redirect URI configurable in the DCR (Dynamic Client Registration) helper library while maintaining backwards
compatibility.

Changes:

• Added redirectURI parameter to PerformDCR() function
• Implemented automatic fallback to default https://mcp.docker.com/oauth/callback when empty string is passed
• Added DefaultRedirectURI constant for consumers to reference
• Maintains backwards compatibility: old consumers can pass "" to use default behavior

Why:

• Enables CE (Community Edition) mode to use localhost redirect URIs for OAuth flows
• Allows different deployment modes (Desktop vs CE) to use appropriate callback URLs
• Supports flexible OAuth configurations without hardcoding redirect destinations

API Change:
// Before (hypothetical old signature):
PerformDCR(ctx, discovery, serverName)

// After (backwards compatible):
PerformDCR(ctx, discovery, serverName, "") // Uses default
PerformDCR(ctx, discovery, serverName, "http://localhost:5000/callback") // Custom

Migration Guide for Consumers:
Existing code needs minimal update - just add "" as 4th parameter to use default redirect URI:

• creds, err := oauth.PerformDCR(ctx, discovery, "github")

• creds, err := oauth.PerformDCR(ctx, discovery, "github", "")

Related issue
Enables implementation of mcp-gateway CE mode OAuth flow with localhost callbacks.

[kgprs]

Looks good - added a small comment about a safety check. Thanks!

[kgprs]

When the redirect url is specified, should we add a check that it's either 127.0.0.1 or mcp.docker.com? Might avoid a situation where an attacker passes in their own url

[kgprs]

I think we should drop the localhost since an attacker can override it