feat: add Google Cloud Secret Manager provider (gcsm)

[yhori991]

Add support for Google Cloud Secret Manager as requested in #25.

Features

• Read/write support: Full provider implementation
• Application Default Credentials: Supports local dev, service accounts, and Workload Identity
• Consistent with other providers: Follows the same pattern as keyring, OnePassword, and LastPass
• Migration support: Enables secretspec import dotenv://.env to migrate secrets to GCP
• Project/profile namespacing: Properly uses project and profile parameters for organization

Implementation

• URI format: gcsm://PROJECT_ID
• Storage path: secretspec-{project}-{profile}-{key} (reads latest version)
• Uses google-cloud-secretmanager-v1 crate for GCP API access
• Feature-gated with --features gcsm to keep binary size minimal
• Documentation matches standard provider format

Usage Example

# Authenticate (first time)
$ gcloud auth application-default login

# Build with GCSM support
$ cargo build --features gcsm

# Set a secret
$ secretspec set DATABASE_URL --provider gcsm://my-gcp-project

# Secrets stored as:
# secretspec-myapp-default-DATABASE_URL
# secretspec-myapp-default-API_KEY

# Run with secrets from GCP
$ secretspec run --provider gcsm://my-gcp-project -- npm start

Fixes #25

[domenkozar]

This should be required given that the code expects it always to be set and errors out

[yhori991]

So true. Thanks for pointing it out. It won't work when we don't have project_id by any means.
I'll change it to a required field.