Remove hard-coded `/api` path

[aryzing]

Would be great if the library could be more flexible to accomodate APIs not deployed at /api. Currently, this line in the code,

https://github.com/mempool/mempool.js/blob/d875bc250de0f110830042763554200fafcb0b41/src/services/api/index.ts#L19

assumes any API the lib interacts with is deployed at /api. Would recommend to either remove it or have it as a param.