added support for mustache scripting of rollup.target_index field

[petardz]

Issue #, if available: 61

Description of changes:

Added capability to use mustache scripting as target_index field value. Only supported param right now is source_index: target_index: rollup_{{ctx.source_index}}

CheckList:

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[downsrob]

Looks like the test and build workflows are failing due to code style errors.
You can run ./gradlew detekt to run this code style workflow locally. I think the ones you added are:

VarCouldBeVal - [targetIndexResolvedName] at /home/runner/work/index-management/index-management/src/main/kotlin/org/opensearch/indexmanagement/rollup/action/mapping/TransportUpdateRollupMappingAction.kt:63:9
VarCouldBeVal - [targetIndexResovledName] at /home/runner/work/index-management/index-management/src/main/kotlin/org/opensearch/indexmanagement/rollup/action/mapping/TransportUpdateRollupMappingAction.kt:118:9
VarCouldBeVal - [targetIndexResolvedName] at /home/runner/work/index-management/index-management/src/main/kotlin/org/opensearch/indexmanagement/rollup/RollupMapperService.kt:73:9
VarCouldBeVal - [targetIndexResolvedName] at /home/runner/work/index-management/index-management/src/main/kotlin/org/opensearch/indexmanagement/rollup/RollupIndexer.kt:127:13

Which shouldn't be hard to fix

[downsrob]

We allow source index names to have wildcards, so we can have scenarios like this:
source_index: logs_etc_*
target_index: {{ctx.source_index}}rollup
At index time the target index would then be resolved to logs_etc*_rollup and then the indexing would fail because you would be trying to write to an index with a wildcard in the name. It is also a bit of an ambiguous customer experience, as a user might expect the target index in this case to be something like logs_etc_1_rollup, logs_etc_2_rollup, etc. based on the actual source index.

To prevent these failures at runtime and clarify the behavior let's validate that the user isn't using both wildcard source indices and a template in their target index.

I think an easy way would just be to temporarily resolve the target index name in the init block of Rollup.kt and validate that it doesn't contain any wildcards. I don't mean updating the target index name with the resolved template, just resolving here to perform input validation. Feel free to implement it however you find best though!

[downsrob]

Thanks Petar, I am happy to approve after these.