FR: Add new field attribute "skip_serializing_when"

[hxzhao527]

similar as #818.
but this allows different values for the same type.
for example

#[derive(Debug, serde::Deserialize, serde::Serialize)]
struct TCP {
    #[serde(skip_serializing_when = true)]
    no_delay: bool,
    #[serde(skip_serializing_when = true)]
    keep_alive: bool,
    #[serde(skip_serializing_when = false)]
    prefer_ipv4: bool,
}
#[derive(Debug, serde::Deserialize, serde::Serialize)]
struct Mux {
    enabled: bool,
    #[serde(skip_serializing_when = 8)]
    concurrency: u32,
    #[serde(skip_serializing_if = 60)]
    idle_timeout: u64,
}

#[derive(Debug, serde::Deserialize, serde::Serialize)]
struct Router {
    enabled: bool,
    bypass: Vec<String>,
    proxy: Vec<String>,
    block: Vec<String>,
    #[serde(skip_serializing_when = "proxy".to_string())]
    default_policy: String,
    #[serde(skip_serializing_when = "as_is".to_string())]
    domain_strategy: String,
    geoip: String,
    geosite: String,
}

[jberryman]

This is nice, because currently serde has no happy path for users who want to skip serializing certain values (to save bandwidth, say) and also want Serialize/Deserialize to be a correct-by-construction roundtrip (I don't want to write unit tests for this, though serde offers enough footguns that maybe its necessary anyway).

If there were an attribute based around Default that skipped serializing that would be okay too, but I would rather just annotate specific fields like in the example above