[config] stop merging arrays from multiple configs when using the dev cli

[mistic]

Pierre merged #161884 in order to fix this when reading from raw config files.
However in dev cli this problem is still happening. We've tracked it down to this line https://github.com/elastic/kibana/blob/main/src/cli/serve/serve.js#L138 which is doing the merge via a simple lodash.merge call. The fix for that would be to use the same merge function / logic implemented in the mentioned PR above.

The problem could be replicated by using this config file https://github.com/elastic/kibana/blob/main/config/serverless.security.yml#L11 and then the following start dev command:

node scripts/kibana --dev --no-dev-config --no-dev-credentials --serverless=security --csp.strict=false --csp.warnLegacyBrowsers=false --elasticsearch.password=changeme --elasticsearch.username=kibana_system --env.name=development --logging.appenders.deprecation={"type":"console","layout":{"type":"json"}} --logging.loggers=[{"name":"elasticsearch.deprecation","level":"all","appenders":["deprecation"]}] --xpack.securitySolutionServerless.productTypes=[{"product_line":"security","product_tier":"essentials"}]

[delanni]

Found a fairly simple way of solving this by a customizer (https://lodash.com/docs/#mergeWith) function - #163928

[delanni]

@mistic can you help me decide if we want to backport this to older kibana versions? I think I was afraid to backport it because this is breaking the CLI's behavior, which is a public API of sorts.

[mistic]

@delanni I think we can backport this at least to 8.10 as the original PR from Pierre is on that branch

[delanni]

I think main was on 8.10 at the time when I merged this, so we're good. I just checked and the code is there in 8.10.