Move `lemmy_api_common` structs to view crates, make a `lemmy_api_utils` crate

[dessalines]

No client devs only need the api structs which are defined in api_common. See lemmy-client-rs for example. Anyway it makes sense to move the api structs like you say. Then lemmy_api_common would be moved to another path like lemmy_api_utils for all the utility functions. And lemmy_api_common would be a new crate which only contains pub use for all the api structs for use by Rust clients.

This PR can be merged already to speed up compilation, and further changes made in separate PRs.

cc @SleeplessOne1917

Originally posted by @Nutomic in #5640 (comment)

[SleeplessOne1917]

It would definitely be nice to separate API structs into their own case, especially if this allows us to only use the logic someone using these structs cares about (e.g. avoiding db related dependencies, not needing to worry about the DbUrl type, deriving Default and some serde types.).

[Nutomic]

avoiding db related dependencies

For this run cargo tree -p lemmy_api_common, look for any unnecessary dependencies and make them optional so they are only compiled with feature full.

not needing to worry about the DbUrl type, deriving Default and some serde types

This could by done with a new api feature flag, for example:

#[cfg_attr(feature = "api", derive(Default, Serialize, Deserialize))]
pub struct MyStruct {
    #[cfg(feature = "api")]
    pub url: Url,
    #[cfg(not(feature = "api"))]
    pub url: DbUrl,
    ...
}