[REQ][Go] Support case-insensitive string enums

Is your feature request related to a problem? Please describe.

Given the following enum spec:

enum:
- HDD
- SSD
- NVME
type: string

The Go client generator will generate an enum that does strict == comparisons, so if the specified API returns NVMe instead of NVME, the response will fail validation because "NVMe" != "NVME".

Describe the solution you'd like

The Java generator supports a useEnumCaseInsensitive option; when that option is enabled, the generator will use case-insensitive equality checks for string enums. Copying that feature into the Go generator would enable generator users to opt in to case-insensitive comparisons for string enums.

Describe alternatives you've considered

Things I've tried:

• First I tried adding NVMe to the list of enum items, but that didn't work because the generated Go enums use constants where the name is the uppercased enum value, so both NVMe and NVME end up with the name MYENUM_NVME, leading to an error MYENUM_NVME redeclared in this block.
• Then I tried using custom templates to implement x-enum-as-string support. That's feasible with custom templates, but the extent of the template changes convinced me that that support would be better implemented in the generator than as a template-only change.

Additional context

Start with the Go generator's string-enum generation and compare it with the Java generator's useEnumCaseInsensitive option. Review the generated Go enum behavior and any custom-template approach mentioned in the issue; done means an opt-in setting produces case-insensitive comparisons for Go string enums without enum constant name conflicts.