CommandInputRecordField needs `default` field

`cwltool` behaves the default value of optional boolean parameters as `false` (I am not sure it is described in the spec) but sometimes we want to set `true` as default for several parameters.
We can use `default` field for the usual parameters but it is not provided for `CommandInputRecordField`.

Is there a way to set the default value for exclusive parameters or am I missing something?

---
Here is a simple example (`wc.cwl`) to show what I want.
It supports the following exclusive options:
- `help`: a parameter to show the help message
- `version`: a parameter to show the version
- `bytes`, `chars`, `lines` and `input`: parameters for usual case

How to write to set the default value of `bytes` `true`?

```cwl
cwlVersion: v1.0
class: CommandLineTool
requirements:
DockerRequirement:
dockerPull: ubuntu
baseCommand: wc

inputs:
mode:
type:
- type: record
name: help-mode
fields:
help:
type: boolean
inputBinding:
prefix: --help
- type: record
name: version-mode
fields:
version:
type: boolean
inputBinding:
prefix: --version
- type: record
name: main-mode
fields:
bytes:
type: boolean?
inputBinding:
prefix: -c
chars:
type: boolean?
inputBinding:
prefix: -m
lines:
type: boolean?
inputBinding:
prefix: -l
input:
type: File
inputBinding: {}
outputs: []
```

Note: #428 is a similar but different issue. #428 is for the default value of record type but this issue is about the default value of specific fields in record object.

Start with the `wc.cwl` example and compare this request with issue #428, then check how `cwltool` currently handles optional boolean fields in `CommandInputRecordField`. Done should define and implement a clear way to express field-specific defaults, with the example demonstrating `bytes` defaulting to `true`.

Written by the indexing model from the issue text.