[JS] Allow auth.accessToken to be either function or string

[kuskoman]

I am using "custom" auth method (basically I need to generate new JWT every request). The easiest way to do it is to use custom accessToken function instead of string. My idea for solving this problem came from similiar code in typescript-axios client:

openapi-generator/samples/client/petstore/typescript-axios/builds/es6-target/api.ts

Lines 281 to 286 in 8c78f13

	if (configuration && configuration.accessToken) {
	const localVarAccessTokenValue = typeof configuration.accessToken === 'function'
	? await configuration.accessToken("petstore_auth", ["write:pets", "read:pets"])
	: await configuration.accessToken;
	localVarHeaderParameter["Authorization"] = "Bearer " + localVarAccessTokenValue;
	}

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[macjohnny]

LGTM