[cli] add template for custom asset source plugin

[vicmeow]

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Does this change require a documentation update? (Check one)

• Yes
• No

Current behavior

Description

Adds a template that makes it easier to get started with creating a custom asset source plugin!

Note for release

• Create asset source plugins from the CLI with sanity init plugin and select the custom asset source option

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made

[kmelve]

Great work!

It would be nice if you included a button with the props.onSelect() handler with an example image in the plugin template. That way developers can get a feel for how it actually works right away.

[skogsmaskin]

Great @vicbergquist! Yes, would be nice with a actual working one. There is a simple example in the docs with a Octocat image.

[vicmeow]

Great work! It would be nice if you included a button with the props.onSelect() handler with an example image in the plugin template. That way developers can get a feel for how it actually works right away.

Great @vicbergquist! Yes, would be nice with a actual working one. There is a simple example in the docs with a Octocat image.

Done!

[kmelve]

LGTM!