Allow an optional proxy

[akirk]

What is this PR doing?

Instead of removing the proxy support #711, this (tempoarily?) brings back the ability to set it.

What problem is it solving?

In Translate Live, we use code from a Github branch that is used by a customized proxy.

How is the problem addressed?

Re-add setPluginProxyURL code so that a proxy URL can be set and if one is set, so that it's used.

Testing Instructions

The instructions of #711 still need to work, i.e. downloads.wordpress.org is to be used directly:

To test, go to http://localhost:5400/website-server/?theme=pendant&plugin=gutenberg and confirm that both the Pendant theme and the Gutenberg plugins are installed, and also that the zip files are downloaded directly from wordpress.org

Additionally, setting a proxy should have that be used.

[adamziel]

@akirk could you instead use a url resource with a full URL of your proxy endpoint?

[akirk]

@adamziel Not sure what you mean, the URL (specifically https://github.com/GlotPress/GlotPress/archive/refs/heads/local.zip) doesn't have the right CORS headers.

[adamziel]

@akirk I meant this:

{
    step: 'installPlugin',
    pluginZipFile: {
        resource: 'url',
        slug: 'https://translate.wordpress.org/plugin-proxy?plugin=glotpress-local.zip',
    }
}

[akirk]

Nice, that wors works, just slightly different (url key instead of slug):

{
    step: 'installPlugin',
    pluginZipFile: {
        resource: 'url',
        url: 'https://translate.wordpress.org/plugin-proxy?plugin=glotpress-local.zip',
    }
}

This means this PR is moot. Thank you!