Match metadata titles to Commerce

[meker12]

@ankumalh A couple suggestions to improve the metadata form in the boilerplate repo to have parity with existing Commerce image management:

Suggestion 1:

• Update the values for the roles to match the names in Adobe Commerce? - thumbnail, base_image, swatch, small_image

(Commerce doc reference Image roles)

Suggestion 2:

• Add labels for position and roles field in the metadata form:

The role isn't too bad since it says: "Choose usage for image" -- but text might be better to say "Select role for image usage"

[ankumalh]

@meker12 - Pablo took care of 1 a while back with https://github.com/ankumalh/assets-commerce/commit/f693023aacd2e404edb285941996f0b627996161

the image in ReadMe needs to be updated though