docs: clarify Gson classpath requirement for fromJson overloads

[cb-karthikp]

• Adds a documentation note in README.md under dependencies about parsing helper overloads.
• Clarifies that some SDK APIs expose Gson-typed overloads (e.g., fromJson(JsonObject)) alongside fromJson(String).
• States that Gson must be present on compile classpath when consumers compile against Gson-typed overloads.

[snyk-io]

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