Return token_strings in the response for Tokenize

[mkozakov]

The API response of tokenize now includes a token_strings field. This PR adds it to the SDK.

Example:
cohere.Tokenize({"hello world"})

&TokenizeResponse{
	Tokens:       []int64{33555, 1114},
	TokenStrings: []string{"hello", " world"},
}

[lfayoux]

lgtm!