core: introduce API to encode return values as response objects

[mkouba]

• resolves Add API to encode aritrary objects as Content and ResourceContents #85

We introduce several encoders that can be used to encode any return value to a relevant response type.

Specifically, there is ToolResponseEncoder that could be used to encode any return value to a ToolResponse; ContentEncoder to encode any return value to Content; ResourceContentsEncoder to ResourceContents and PromptResponseEncoder to PromptResponse.

We also register JsonTextContentEncoder and JsonTextResourceContentsEncoder by default. As a result, any "custom" return value returned from a @Tool method is encoded as TextContent containing the value encoded as JSON (using Jackson), and any "custom" return value returned from a @Resource method is encoded as TextResourceContents containing the value encoded as JSON (using Jackson).

[geoand]

Nice!