Allow customized User_Agent used by lambda-runtime-api-client

[bnusunny]

In AWS Lambda Adapter, we want to send a custom User_Agent to lambda Runtime API.

lambda-runtime-api-client has a default User_Agent string, e.g. "aws-lambda-rust/0.5.0". To allow custom User_Agent, we can set an environment variable CUSTOM_USER_AGENT at compiling time, like this.

const CUSTOM_USER_AGENT: Option<&str> = option_env!("CUSTOM_USER_AGENT");

And when building the http request, use its value if it is configured, otherwise send the default User_Agent. e.g.

pub fn build_request() -> http::request::Builder {
    let user_agent = match CUSTOM_USER_AGENT {
        Some(value) => value,
        None => USER_AGENT,
    };
    http::Request::builder().header(USER_AGENT_HEADER, user_agent)
}

@calavera @nmoutschen What's your opinion?

[calavera]

That makes sense to me. I'd make the variable name LAMBDA_RUNTIME_USER_AGENT to be explicit on what's used for.

[rimutaka]

1. What is the use case for this? Just curious.
2. user_agent can also be a constant with the match statement that builds it

[bnusunny]

Good suggestions. I will send a PR shortly.

This will help us to understand the usage of different runtimes/custom clients.