OpenApi 3.1: external resolution of Example Object `externalValue`

[char0n]

Specification: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#example-object

We have to guarantee mutual exclusivity between value and externalValue fields.

externalValue with URI pointing to a resource that is either JSON or YAML will be parsed into ApiDOM. Other types of content will be escaped using base64 encoding and set into value field.

[char0n]

Needed to implement BinaryParser which returns base64 encoded data within ParseResultElement. More here: #468