feat: add portal CRUD commands (get, create, update, delete) (#674)

[saiflayouni]

Summary

• Adds the missing create, get, update, and delete operations for Apigee integrated developer portals
• Previously only list was supported, making portal creation and management a UI-only operation
• create accepts a JSON file describing the portal and an optional --site flag to set a specific portal ID
• update does a full replace (PUT) using a JSON file
• delete removes the portal by site ID

New commands:

# Get a portal
apigeecli sites --org $ORG get --site $SITE_ID

# Create a portal from a JSON file
apigeecli sites --org $ORG create --file portal.json [--site $SITE_ID]

# Update a portal (full replace)
apigeecli sites --org $ORG update --site $SITE_ID --file portal.json

# Delete a portal
apigeecli sites --org $ORG delete --site $SITE_ID

Fixes #674

Test plan

• Run apigeecli sites create --file portal.json and verify the portal is created
• Run apigeecli sites get --site <id> and verify the response matches what was created
• Run apigeecli sites update --site <id> --file updated.json and verify the change
• Run apigeecli sites delete --site <id> and verify the portal is removed
• Run apigeecli sites list to confirm the portal no longer appears

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