Optional create auth and gas tank service

[boqiu]

This change is 

[Copilot]

Pull request overview

This PR updates service initialization and API routing to make the AccountAbstract (auth) and GasTank services optional based on configuration, and updates the example environment file accordingly.

Changes:

• Make service.New only construct AccountAbstract and GasTankPaymaster when corresponding contract addresses are configured.
• Conditionally register /aa/auth/* and /aa/gastank/* routes only when those services are enabled.
• Update .env.example to document how to enable/disable these optional services (and adjust default rate-limit values).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
service/service.go	Builds Services with optional AccountAbstract and GasTank based on configured addresses.
api/route.go	Registers AA/GasTank routes only when the services are non-nil; adjusts default rate limits.
.env.example	Documents optional enabling via env vars; comments out GasTank address by default.

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