Document input format for data sources

Currently options are as follows:

#[derive(Clone, Copy, Debug, Default, Eq, PartialEq, Serialize, Deserialize, utoipa::ToSchema)]
#[serde(rename_all = "snake_case")]
pub enum SourceInputFormat {
    #[default]
    Json,
    OtlpLogsJson,
    #[serde(alias = "otlp_logs_proto")]
    OtlpLogsProtobuf,
    #[serde(alias = "otlp_trace_json")]
    OtlpTracesJson,
    #[serde(
        alias = "otlp_trace_proto",
        alias = "otlp_trace_protobuf",
        alias = "otlp_traces_proto"
    )]
    OtlpTracesProtobuf,
    #[serde(alias = "plain")]
    PlainText,
}

Start from the SourceInputFormat enum shown in the issue and review its variants, serde names, and aliases. Document the accepted input formats for data sources, including JSON, OTLP logs and traces, protobuf, and plain text; done means users can identify the supported names and aliases.