Remove `Hashable` from `ServerSentEvent` and `ServerSentEventWithJSONData`

### Motivation

In `ServerSentEventWithJSONData`, the generic `JSONDataType` may not guaranteed to conform to `Hashable`.

For example, there are a lot of packages can be found on GitHub for `JsonSchema`, and they all do not conform to `Hashable`.

And, the decoder or deserializer may not need Hashable constraint.

some packages list below:
1. https://github.com/kylef/JSONSchema.swift/blob/master/Sources/JSONSchema.swift
2. https://github.com/objecthub/swift-dynamicjson/blob/main/Sources/DynamicJSON/JSONSchema/JSONSchema.swift

### Proposed solution

Remove `Hashable` from `ServerSentEvent` and `ServerSentEventWithJSONData`.

### Alternatives considered

_No response_

### Additional information

_No response_

Search the project for the declarations and uses of ServerSentEvent and ServerSentEventWithJSONData, then inspect how their generic JSONDataType is constrained. Done means both types no longer require Hashable while their decoding or deserialization use remains valid.

Written by the indexing model from the issue text.