[ENHANCEMENT] ExistingSecretkey for External Elasticsearch

[hamza-m-masood]

Describe the use case:

Customers should be able to specify an existingSecretyKey for tls when connecting to external elasticsearch

Describe the enhancement/feature:

Right now, if customers enable tls and want to provide an existing secret for self-signed certificates, connecting to external Elasticsearch then they would need to use the key externaldb.jks, according to this guide: https://docs.camunda.io/docs/self-managed/setup/guides/using-existing-elasticsearch/

It would be better to have a customizable key for external Elasticsearch tls secret. This would be the yaml path for the new value:
global.elasticsearch.tls.existingSecretKey
Desired outcome and acceptance tests:

[aabouzaid]

Example of the hard-coded key externaldb.jks