Enhancement : Support for "variable-length" file generation via HTTP API

[anujtewari]

Wire Version: v0.9.0

What were you trying to do?
Generate a "variable-length" file via HTTP API.

What did you expect to see?
Support for "variable-length" file via HTTP API.

What did you see?
No support right now via HTTP API.
As per slack discussion the proposal is expose to a feature to generate "variable-length" file using /files/{fileId}/contents.

One proposal - have a query param type to choose between fixed and variable(can use better names).
Based on this value, appropriate writer can be used. i.e.

for fixed -> use what we have wire.NewWriter(w)
for variable -> use

wire.NewWriter(w, wire.VariableLengthFields(true), wire.NewlineCharacter(""))

** One other thing to take care is how the writer currently handles
{3600}BusinessFunctionCode - TransactionTypeCode. This field is optional as per spec and when not set in payload while creating(/files/create) the file, the decoder still adds the field with "". This happens because a string in Go cannot be nil. Any string field where the JSON key isn't present will end up as an empty string in Go.
This ends up adding * to this tag element for when we request file content i.e CTR* but should be just CTR here.
@atonks2 will investigate and address this after understanding its impact.

[atonks2]

Thanks @anujtewari! I'll break the BusinessFunctionCode question into its own issue so we can track these separately.

[atonks2]

Extracted the BusinessFunctionCode question to #240