Support `any` type values in log/event attributes

[dyladan]

Currently all semantic conventions need to declare what type they are (e.g. string, boolean, etc). In some cases such as feature_flag.result.value we may not know what type the value of the attribute is. In the past, body fields were used to solve this use case, but body is being de-emphasized in favor of attributes. Because we don't know the type, the only thing we can currently do is JSON stringify the value and have another attribute that disambiguates the original type (e.g. is an attribute containing string value the string "6" or the number 6?).

I would like to be able to declare the type of feature_flag.result.value to be any/anyvalue/undefined/unknown type (i'm fine with any name), indicating that the type of the value is unknown. The type information is carried in OTLP as part of the anyvalue structure, so it should be possible to determine on the backend.

This addresses part of open-telemetry/semantic-conventions#1669 but does not completely solve the complex attribute problem which will need to be addressed later.

We would like to use it in open-telemetry/semantic-conventions#1990