[CLI] Create `cg add field` command

[widal001]

Adding a custom field

User story

As a developer defining a CommonGrants API, I want to add a new custom field with configurable options, so that I can extend the API schema easily.

Developer experience

Simplest use case: Add a custom field by specifying name and type.

cg add field <name> <type>

Additional Features

• Provide an example value with --example.
• Add a description using --description.

Example Usage

# Add a simple custom field
cg add field fundingAmount number

# Add a custom field with an example value
cg add field fundingAmount number --example 100000

# Add a custom field with a description
cg add field fundingAmount number --description "The total amount of funding available"

Technical details

• The command should append the new field to the appropriate schema definition in the TypeSpec project.
• If --example or --description is provided, they should be included as metadata in the schema definition.

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[widal001]

We've decided to descope this command for now. We may re-open this ticket if there's a desire to support this in the future