Version constraints between Vault and its plugins

[calvn]

Feature Request:

Implement a system to define plugin version constraints. This is useful on cases where plugin.HandshareConfig.ProtocolVersion has not changed, but a patch has been implemented in either Vault or a plugin.

Sample scenario: If a plugin (e.g. 0.1.0) has been updated to have a change in behavior (a change that applies to Vault 0.9.0+), but being ran under an older version of Vault (e.g. 0.8.3), core might not respect those changes. Both Vault versions are using ProtocolVersion: 3, and bumping this for a single plugin patch is non-ideal since it would force all other plugins to be rebuilt as well. In this scenario, Vault (0.8.3) would constraint the version of the plugin to be less than the version that implemented this change in behavior. On the other hand, if a newer version of Vault is being ran (0.9.0+), the constraint might be that the plugin needs to be 0.1.0 and above.

cc @briankassouf

[vishalnayak]

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[heatherezell]

@calvn is this issue still valid? Please let me know. :)