cmd: enhance snake_case generation

[hgiasac]

• Change the https://github.com/huandu/xstrings library to enhance the snake_case generation. The existing library doesn't work well with digits after characters, e.g., base64 => base_64.
• Allow users to configure the naming style with the HASURA_PLUGIN_CONNECTOR_NAMING_STYLE environment variable

[github-actions]

Package	Line Rate	Health
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go/command/internal	83%	✔
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go/command	34%	❌
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go	0%	❌
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go/version	43%	➖
github.com/hasura/ndc-sdk-go/connector	54%	➖
github.com/hasura/ndc-sdk-go/example/reference	57%	➖
github.com/hasura/ndc-sdk-go/ndctest	51%	➖
github.com/hasura/ndc-sdk-go/scalar	75%	✔
github.com/hasura/ndc-sdk-go/schema	48%	➖
github.com/hasura/ndc-sdk-go/utils	35%	❌
Summary	52% (6735 / 12954)	➖

Minimum allowed line rate is 40%