refactor: fix FieldMayBeFinal warnings across modules

[akenra]

Hi @olegz!

I wanted to give the project a bit more love and hopefully help revive the somewhat stalled maintenance pace, starting with simple cosmetic changes.

Specifically, I went through the IntelliJ FieldMayBeFinal inspection warnings and fixed them across the codebase. Most changes are straightforward final additions, though a few required a bit more care:

• SimpleFunctionRegistry.wrappedFunctionDefinitionsCacheSize - removed the field entirely. The value is only used in the anonymous LinkedHashMap.removeEldestEntry(), so the constructor parameter is captured directly
• Made cache size configurable via spring.cloud.function.registry.cache-size property (default 1000) since the old test used reflection on a now-removed field
• Renamed private static final fields that didn't follow UPPER_CASE convention (breeds→BREEDS, names→NAMES, results→RESULTS, webflux→WEBFLUX, errorAttributes→ERROR_ATTRIBUTES, headers→HEADERS)

Let me know if anything needs adjustments!