[feat]: add custom headers for the strapi.client http

[AbderrahmaneMouzoune]

A clear and concise description of what the feature is

Have the opportunity to add custom headers when request strapi

Why should this feature be included?

Can be usefull for specific use case like only querying draft entity or interact with plugins

Please provide an example for how this would work

import { strapi } from '@strapi/client'

export const strapiDraftClient = strapi({
  baseURL: strapiGetUrl(),
  auth: env.STRAPI_API_TOKEN,
  headers: {
    'X-Preview-Mode': 'true',
  },
})

[AbderrahmaneMouzoune]

Btw here's my proposal : PR #76