Add support for capturing HTTP Client request bodies

[JonasKunz]

Our agent has the capability to capture HTTP request bodies for server frameworks (e.g. servlet api).
A requested feature is to also capture the first few (e.g. 1024) bytes of HTTP client request bodies aswell:

• Add request body capturing support for Apache HTTP Client v4 Capture request body for Apache HttpClient 4.x #3692
• Add request body capturing support for HttpUrlConnection Capture request body for HttpUrlConnection #3724
• Add request body capturing support for Spring WebClient Add body capturing for reactive spring webclient #3754
• Add request body capturing support for Apache HTTP Client v5 Add support for apache http client v5 body capturing #3767
• Decide on how the data is tranferred to APM-server (e.g. dedicated IntakeV2 field or otel attirbute) and how it is stored
  - we decided to keep it in labels for now as the feature is experimental anyway
• Respect the charset from the Content-Type header when converting the bytes back to strings Perform charset-aware decoding of request bodies #3726