[Transform] Field naming is inconsistent between _stats, HLRC and _cat

[hendrikmuhs]

As #53776 shows the naming of some fields is inconsistent between the ouput of _stats, the HLRC getters in code and what _cat/transform returns.

The problem seems to originate from HLRC inheriting the getters from the shared code IndexerStats object. However, we renamed some fields which in the server code is benign.

• fix naming in HLRC so its consistent with field naming of xcontent
• use consistent naming of fields in _cat/transform
• (optional) have consistent getters/setters in the server code (optional, because its non-public, however it's good to have it consistent)

(This is non-breaking if we keep the old getters/setters and only add new ones)

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)