[Feat]: Does not support file uploads

### Is your feature request related to a problem? Please describe.

Many A2A implementations (and the spec) support/require file uploads from the A2A client to A2A server. Currently the a2a-inspector does not support any way to upload a file.

### Describe the solution you'd like

Provide a file-upload mechanism in addition to text chat.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

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