OAS number - decimal should map to float128 in Go

[baywet]

See this PR for the mappings across languages MicrosoftDocs/openapi-docs#121

Currently decimal in Go maps to float64, which is too short. It should map to float128 instead. This will be a breaking change and requires:

• A new method in ParseNode
• A new method in SerializationWriter
• Implementations across serialization/parse node
• A new entry in the Go Conventions Service (kiota)
• Potential new mappings in Go CodeMethodWriter for the serialization and parse node calls.