Add config for supported features

[michaelhall923]

Need config options in nostr.json outlining which of the optional features are currently supported.

Filename

Specify whether variable filenames are supported

Type Conversion

Must specify which specific conversions are supported (i.e. jpg to png)

Query params

Specify supported sizes (if any), whether custom w/h are supported, and which aspect ratios are supported.

Any config options not specified by the host should be assumed to be unsupported.