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,

mempool.js/src/services/api/index.ts

Line 19 in d875bc2

baseURL: `${protocol}://${hostname}${network}/api/`,

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