feat: [v2] Use a safe type for env override names

[siegfriedweber]

Description

Use a safe type for env override names

env_overrides in v2::role_utils::CommonConfiguration is now the new v2::env_overrides::EnvOverrides type (a BTreeMap<EnvVarName, String>) instead of a HashMap<String, String>, so environment variable names are validated on deserialization and kept in a deterministic order. v2::role_utils now defines its own CommonConfiguration, Role and RoleGroup instead of re-exporting them from crate::role_utils.

Example: stackabletech/hive-operator#748

Part of stackabletech/internal-issues#167

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