Clarify "format": "byte" vs "format": "base64" in OAS 3.0.x

[handrews]

See PR #3167 for a lot of context.

In OAS 3.0.3:

• §4.4 Data Types shows format: byte for base64-encoded binary data, matching the same section in OAS 2.0
• §4.7.14.3 Considerations for File Uploads has a very minimal example showing format: base64 alongside an example of format: binary; there is no sign of format: byte
• §4.7.14.5 Special Considerations for multipart Content mentions format: base64 alongside format: binary when talking about default Content-Type values, but does not mention format: byte

It's clear that format: byte was intended to carry over from OAS 2.0, but that base64 is more intuitive and got used everywhere outside of the table of formats.

@OAI/tsc I feel like the most sensible thing to do here would be to define both byte and base64 as having the same effect, and note that byte is compatible with OAS 3.0, while base64 is analogous to using contentEncoding: base64 in OAS 3.1.

I'm guessing that adding a format would not be allowed by SemVer, but we could make the argument that the current situation is so confusing that defining both is just a clarification.