Add pluggable extension point for TranslogDeletionPolicy and move CCR specific logic to plugin.

[mch2]

#1100 added translog pruning logic behind an index setting that is specific to CCR. This logic and setting should be scoped only to the CCR plugin.

Suggested solution:
We need to create an extension point for plugins to provide their own TranslogDeletionPolicy logic. Right now we have the limitation that only one plugin may override engine (A suggestion to break this limitation in #1254 ). Until that is available, we should restrict only a single plugin from overriding deletion policy.

[adnapibar]

Instead of creating a separate plugin API for this, we can modify EnginePlugin to provide the extension points. Nick came up with an idea of creating an EngineConfigFactory to solve the issue.

[adnapibar]

Reopening it for 1416

[saikaranam-amazon]

@adnapibar Thanks for doing these changes.

Instead of creating a separate plugin API for this, we can modify EnginePlugin to provide the extension points

Does this mean that the only one plugin can override Engine? If that is the case, we already have use-case to override Engine by two plugins - `CCR` and `k-NN` and as only one plugin can override engine, these will need additional changes in the related plugins to avoid the conflict.

Individual plugins can still override the translog deletion policy.

[nknize]

Individual plugins can still override the translog deletion policy.

Correct. This avoids the Engine override conflict and enables CCR and KNN to co-exist. This design maintains that only one plugin can override the translog deletion policy. We've discussed the possibility of adding a merge that enables multiple plugins to define min gen criteria; but that's out of scope for now.