Include a variable to control synthetic_source_keep parameter

[salvatore-campagna]

This PR introduces a new track parameter, synthetic_source_keep which is used to control the
behaviour of synthetic source for all field types. It can have values none, arrays or all (all
not usable when set at index level).
See elastic/elasticsearch#112706 to understand the effect of each value.

Later on we will use this to change the behaviour in our nightlies and run benchmarks on both elastic/logs
and elastic/security using value arrays.

This needs backporting to 8.15

[kkrik-es]

all is not allowed at the index level, as it effectively disables synthetic source. The only values are none, same as default, and arrays.

[salvatore-campagna]

I see

public static final Setting<SourceKeepMode> SYNTHETIC_SOURCE_KEEP_INDEX_SETTING = Setting.enumSetting(
        SourceKeepMode.class,
        "index.mapping.synthetic_source_keep",
        SourceKeepMode.NONE,
        value -> {
            if (value == SourceKeepMode.ALL) {
                throw new IllegalArgumentException("index.mapping.synthetic_source_keep can't be set to [" + value.toString() + "]");
            }
        },
        Setting.Property.IndexScope,
        Setting.Property.ServerlessPublic
    );

[github-actions]

💔 All backports failed

Status	Branch	Result
❌	8.15	Backport failed because of merge conflicts

Manual backport

To create the backport manually run:

backport --pr 682

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details