[feature] - add namespace as environment variable to allocations

[henrikjohansen]

In the spirit of #5554 it would be hugely appreciated if the namespace name that a particular job runs in could be made accessible inside it's allocations environment, ie :

NOMAD_NAMESPACE=default

Reasons for having this include changing application behaviour based on environment (think test vs. production) and tagging of logs and metrics with the namespace name in order to facilitate easier multi-tenant operations.

[henrikjohansen]

Implemented in #6192.

[github-actions]

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