feat: Support stream init diagnostics events for FDv2.#133

kinyoklion wants to merge 5 commits intomainlaunchdarkly/java-core:mainfrom

rlamb/SDK-1632/FDv2-Stream-Diagnosticslaunchdarkly/java-core:rlamb/SDK-1632/FDv2-Stream-DiagnosticsCopy head branch name to clipboard