Support alter from manual to auto without specifying scheduler mode

[noCharger]

Description

Support alter from manual to auto without specifying scheduler mode. In detail:

Step 1: Create a manual refresh index (full or incremental)

Step 2: Alter it by set auto_refreh=true, by default it will switch to auto refresh mode and use external scheduler when spark.flint.job.externalScheduler.enabled = true

Check List

• [x] Implemented tests for combination with other commands
• [x] Commits are signed per the DCO using --signoff

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[noCharger]

What happens in these 2 scenarios?

Scenario 1

1. create index with "auto_refresh": true and "scheduler_mode": "external"
2. alter index with "auto_refresh": false
3. alter index with "auto_refresh": true

Scenario 2

1. create index with "auto_refresh": true and "scheduler_mode": "internal"
2. alter index with "auto_refresh": false
3. alter index with "auto_refresh": true

Is it intended that in both scenarios the resulting index should be auto refresh index using external scheduler? Does the condition check in isSchedulerModeChanged cover this case? I believe originalOptions will still have "scheduler_mode": "internal"

If scheduler_mode is already exist in original index option, it should copied to the updated index and behave as of expacted, hense Scenario 2 should be internal instead. The case to cover in this PR is when original index option doesn't have scheduler_mode field.