Improvements to Message in the Oracle Program

[Reisen]

This PR extends the Message type with features that are helpful to users consuming the program as a library.

• Adds strum/serde to Message* during --features = library use.
• Adds methods to extract id/publish_time from any message.
• Exports the new MessageType enum generated by strum.

[guibescos]

What is the reason for having two "subfeatures" serde and strum.
Instead of using #[cfg(feature = "library")] in the code

[Reisen]

Just for the flexibility, It's nice in cases where for some reason one of these packages creates dependency conflicts with something the implementer is using, in reality most of the time someone is going to want both and will not have problems and will indeed just use library but this allows for this pattern:

// Strum only
pyth-oracle = { features = ["strum"] }

// Or if defaults ever get set:
pyth-oracle = { default-features = false, features = ["strum"] }

Etc.

Note post 1.60 versions of Cargo auto-define the following:

[features]
...
serde = ["dep:serde"]
strum = ["dep:strum"]

[ali-behjati]

well thinking more, no Message will be exported without library feature though, right?

Maybe we can keep serde separate but i was thinking maybe we can hide strum from users entirely :?