[Java][okhttp-gson] Refactor handling of 'number' types in oneOf

[karzang]

This PR improves the handling of number datatypes used in oneOf by removing a previous workaround which treated BigDecimal as a languagePrimitiveType.

This PR fixes #16166 since treating BigDecimal as a primitive results in, the generator not adding any imports for it. Instead this change provides separate handling of number datatypes in the oneof_model.mustache template.

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[karzang]

@cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02)

@wing328

[welshm]

Change looks fine to me - but it's targeting Java Client and OK HTTP - not Spring. Looks like we don't have a Java Client technical committee though

[wing328]

@karzang thanks for the PR. Can you please add a fake endpoint with parameter type number so as to catch the issue moving forward?

e.g. from the issue

		"parameters": [
			{
				"name": "amount",
				"in": "path",
				"required": true,
				"schema": {
					"type": "number"
				}
			},

[wing328]

tested locally to confirm the fix. will file a PR to add a test after merging this PR

update: done via #16263