Rename proto files to lower_snake_case

[patriknw]

According to the style guide https://developers.google.com/protocol-buffers/docs/style:

Files should be named lower_snake_case.proto

[patriknw]

and field names, e.g. cart_id

[thothmoses]

We need to make this change in the code - Then I can make the changes in the docs.

[GKuldeepak-Knoldus]

Hi, It would be more easier if you can specify the file name or field_names to change.