feat: add ACCESS_TOKEN in .env.example

[wolfsilver]

Add ACCESS_TOKEN to .env.example and read it in NewService function.

• config.go

  • Add AccessToken field to Config struct.
  • Parse and load ACCESS_TOKEN from the environment in getConfigFromEnv function.

• memogram.go

  • Read ACCESS_TOKEN from the configuration in NewService function.
  • If ACCESS_TOKEN is not blank, split it with :, get userId and access_token, and store them in userAccessTokenCache.

• .env.example

  • Add ACCESS_TOKEN variable in the format userId:access_token.

---

For more details, open the Copilot Workspace session.

Fixes https://github.com/usememos/telegram-integration/issues/25

[boojack]

It's better to use a simple text file to store this data. Implemented with https://github.com/usememos/telegram-integration/commit/5d7dc80cbc4e5076de789a0556e84f29150b4c00