Add support for string encoded frame length header

[solnaranu]

Hello @kpavlov and other contributors,

first, let me thank you for this library. I just started using it and it looks like it's going to help me save a lot of time.

Now, to the issue I had - I'm integrating with a vendor which doesn't use a binary frame length header, but a 4 byte, zero padded, ASCII encoded one. For example, an ISO message of length 93 would have the length header of 0093.

Since the first stage in the ChannelPipeline is Netty's LengthFieldBasedFrameDecoder and it expects a binary length header, I needed to extend it and override it's getUnadjustedFrameLength method in StringLengthFieldBasedFrameDecoder.

Then, I did some config plumbing in ConnectorConfiguration which is used in Iso8583Encoder and Iso8583ChannelInitializer.

Currently, US_ASCII is hardcoded when encoding and decoding, but I'd be willing to make that configurable if it's a realistic use case, which I'm not aware of.

Usage example for the new configuration:

val serverConfiguration = ServerConfiguration.newBuilder()
        .frameLengthFieldLength(4)
        .encodeFrameLengthAsString(true)
        .build

Existing tests are passing, but I haven't added new tests, only tested it manually. I will add tests if this PR has a chance of being accepted. I'd be glad to discuss it and take suggestions.

Cheers,
Bojan

[kpavlov]

Thank you, @solnaranu for your PR. Please add some tests.

[solnaranu]

Thank you, @solnaranu for your PR. Please add some tests.

Will do! I'll try to find some time in the following weeks.