Omit base uri from request token props interface

[YashodipD]

Fixes - #7219

Here, ContainerService.requestToken(props: RequestTokenProps): Promise<TokenProps> interface

When requesting a token for first time baseUri is unknown, hence RequestTokenProps should not force to pass empty string. In fact, we expect same function to return a base uri. we made this change here - https://github.com/iTwin/itwinjs-core/pull/7140