Declare more functions in importlib.metadata.PackageMetadata

Feature or enhancement

Add get(), keys(), and potentially more fields from Mapping to PackageMetadata. Also get_payload().

Pitch

It is common for PackageMetadata usages to incorporate calls like get() and keys(), which the protocol are currently missing.

There is also a style of metadata that uses the payload as the long description. Current tools tend to use get_payload() to retrieve the data (implemented by email.mail.Message), but the function is also missing from the protocol.

Previous discussion

N/A

定位 importlib.metadata.PackageMetadata，并将其协议与 Mapping 和 email.mail.Message 进行比较。检查 get()、keys() 和 get_payload() 的表示方式，然后添加所请求的声明和测试，以确认这些接口可用。