Allow Forceful Mode Restrictions

[pykettk]

Description

Allows developers to forcefully prevent indexers being locked to specific modes via di.xml configuration and extends the deployment configuration validation to enforce these rules.

Discussion Points

1. Is flagging the deployment configuration as invalid over-aggresssive?
2. Should we take it a step further and enforce these rules in the SetIndexerMode plugin too?
   • I'm not sure about this, personally. I feel like locking the mode outside of the deployment configuration file(s) defeats the purpose of this module.

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Closes #11

[peterjaap]

1. No, since we're adhering to Adobe's best practices concerning this.
2. No, the possibility to set (and override) should still be available.