feat: Add support to read provider schema from local file

[tmeckel]

Hello!

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Issue details

The registrygen tool has no option to read a provider schema JSON file from the local disk instead of downloading it from the provider repository.

This slows down the development of providers, as you always have to first upload the schema file into the provider repository in order to check the generation of the provider documentation.

With a flag like --useLocalSchemaFile to the docs command, registrygen will read the file specified by the --schemaFile directly instead of downloading it from the repository.

[tmeckel]

I've a patched version of registrygen ready locally. So I could provide a PR quickly, if my feature request make sense to the Pulumi docs team.

[iwahbe]

CC @cnunciato

[iwahbe]

We are deprecating registrygen (this repo) in favor of github.com/pulumi/registry/tools.