Feature/elastic search

[lorenzomf97]

Mainly implemented correct log querying from Elasticsearch (see below for more details)
Documentation regarding configuration of custom Elasticsearch cluster for this project

FEATURES IMPLEMENTED:

• Managed most common HTTP server and logical errors that can occur while querying Elasticsearch (ES) or due to Spring API itself (e.g., job_id doesn't exist, no hits retrieved, void index, or invalid/deleted index).
• Properly handled multiline log entries in grok filters for Logstash.
• Correctly filtered the "job_id" field in Elasticsearch by enforcing exact matching (added ".keyword" to "job_id" => "job_id.keyword" inside ES queries, following the official Elasticsearch documentation).
• Rewrote Elasticsearch querying using Java ES official mapping instead of the old raw text query.

FEATURES TO BE IMPLEMENTED:

• Implement proper management of ES query results by using offset and limits instead of the temporary "/_search?size=10000" approach to fully benefit from proper ES hits-retrieving pagination (in accordance with ES official documentation).
• Every hostname is treated as "valid" during SSL certificate verification => this has to be fixed.
• Implement value calculation/return for some optional fields returned by the API (currently hardcoded at default or null values), following the OpenEO documentation specifications.
• Make further code improvements.
• Implement logEntry and add it to the logEntries list, then return the result with pagination links (this relates to the first to-do item).

[pierocampa]

Log requests are properly handled, there's just a few italian comments to be translated, and the whole SSL context and host verification part which might probably better removed, letting the default Java implementations to be used.