Add constructor for custom mapping with Jackson

[kostya05983]

Subsystem
ktor-client:ktor-client-features:ktor-client-json:ktor-client-jackson

Motivation
Pr adds custom jackson mapper, now people can do

val client = HttpClient(HttpClientEngine) {
    install(JsonFeature) {
        serializer = JacksonSerializer(ObjectMapper())
    }
}

refs #1208
Link for docs pr https://github.com/ktorio/ktorio.github.io/pull/204
Solution
I deleted one primary constructor and added to constructor in body of class with argument of previous constructor and constructor with objectMapper parameter to init backend value.

[cy6erGn0m]

The only question is why? What kind of mapper do you want to pass instead of the default mapper and why do you need to replace it? Please remember that missing registerKotlinModule invocation may lead to an inability to serialize/deserialize kotlin classes in some cases.

[kostya05983]

I did it to help the user in this ticket #1208. I think, maybe someone uses DI and want to pass objectMapper easy.

[kostya05983]

Hello, @e5l may you review this?
Thanks

[e5l]

Hi @kostya05983, thanks for the PR.

Merged in develop.