[go] Generate inline enum models and prefix all enums

Created by: fantavlik

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CC: @antihax @bvwells @grokify @kemokemo

This aims to address the following issues: https://github.com/OpenAPITools/openapi-generator/issues/535 (also see related closed PR: https://github.com/OpenAPITools/openapi-generator/pull/542) https://github.com/OpenAPITools/openapi-generator/issues/2050 https://github.com/OpenAPITools/openapi-generator/issues/2360

Description of the PR

There are four main aspects:

1. Enums that are defined inline were not being generated (see https://github.com/OpenAPITools/openapi-generator/issues/2360 for an example), this PR follows similar logic to the typescript-fetch generation template to remedy this: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache#L92-L109
2. Enums names often result in namespace collisions within the package being generated (see https://github.com/OpenAPITools/openapi-generator/issues/535), prefixes have been added to help prevent this for most cases.
3. Numeric enum values were improperly being quoted, now only strings are.
4. Numeric enum names were not properly escaping plus/minus signs and decimal points e.g. to differentiate between -1 and 1 or 1.1 and 11