Remove strcase dependency

[rileykarson]

We use a "strcase" library in exactly one place, and it's for a pretty routine conversion to jsonCamelCase. We can include a few LOC in the provider and cut a dependency.

Reference: b/177115695

[melinath]

Resolved by merge of #8296 and hashicorp/terraform-provider-google-beta#2900

[ghost]

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