[Task]: Use file config interpolation for encryption key

[renuka-fernando]

Please select the area the issue is related to

Platform API

Please select the aspect the issue is related to

Aspect/Configuration (Config files, settings, env vars, defaults)

Description

[platform_api.security]
encryption_key = '{{ env "APIP_CP_ENCRYPTION_KEY" }}'

Here instead of env, use file.

Because, users are not generate this and copy and paste, set env var for this, like the Gateway token. We can keep this secret in a file then.