service discovery: add config boolean parameter and fingerprinting

[jrasell]

This change adds a new client parameter which allows operators to control whether the client can utilise the native service discovery feature. This will default to true, meaning the feature is enabled. This configuration value is exported as a fingerprinted attribute under the meta partition.

In order to ensure allocations are only placed on clients which have the feature enabled, when using provider = nomad the job group is mutated to contain a constraint. This mutator mimics the current Vault mutator.

closes https://github.com/hashicorp/team-nomad/issues/265

[hashicorp-cla]

All committers have signed the CLA.

[github-actions]

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