[Feature] Context supports carrying more kinds of context information

## Issue Description

Type: *feature request*

### Describe what feature you want

`Context` represents the context of a local invocation chain in Sentinel. In some circumstances, we may need more context-aware information (e.g. environment flag, business-relevant flag) to do flow control. But currently the `Context` only supports limited information (e.g. `origin`), so the `Context` should be improved to support carrying more kinds of context information.

An intuitive idea is to add a map in `Context` which represents pairs of context attachments. This can generalize all kinds of context information as a unified map, so that we don't have to put objects as fields of `Context`.