fix(fetch): add HTTPS Agent types to fetch & Request

[MichaelDeBoey]

See @cliffordfajardo's remix-run#18

What is the change?

• Add typings for https agent
• Add correct typings fetch's options param

Why are we making this change?

• Allow a user to pass https to `Request
• Allow agent & other RequestExtraOptions values to be passed to fetch

Tests

Type changes only. Ran npm test inside of package/fetch and all tests pass

Test output for `package/fetch`

 247 passing (4s)
  4 pending

-----------------|---------|----------|---------|---------|-----------------------------------------
File             | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                       
-----------------|---------|----------|---------|---------|-----------------------------------------
All files        |   97.15 |    96.79 |   89.32 |   97.15 |                                         
 src             |   96.69 |    96.58 |   86.58 |   96.69 |                                         
  body.js        |   96.97 |    95.45 |   97.36 |   96.97 | 211-212,250-254,263,464-473             
  fetch.js       |   99.46 |    96.38 |     100 |   99.46 | 117-118                                 
  headers.js     |   92.73 |    93.75 |   81.81 |   92.73 | 16-24,29-38,95-96                       
  lib.node.js    |     100 |      100 |     100 |     100 |                                         
  package.js     |     100 |      100 |     100 |     100 |                                         
  request.js     |   95.62 |      100 |   56.25 |   95.62 | ...-170,173-174,178-179,183-184,188-189 
  response.js    |   98.59 |      100 |    90.9 |   98.59 | 59-60                                   
 src/errors      |     100 |      100 |     100 |     100 |                                         
  abort-error.js |     100 |      100 |     100 |     100 |                                         
  base.js        |     100 |      100 |     100 |     100 |                                         
  fetch-error.js |     100 |      100 |     100 |     100 |                                         
 src/utils       |   99.27 |     97.4 |     100 |   99.27 |                                         
  form-data.js   |   98.29 |       92 |     100 |   98.29 | 115-116                                 
  get-search.js  |     100 |      100 |     100 |     100 |                                         
  is-redirect.js |     100 |      100 |     100 |     100 |                                         
  is.js          |     100 |      100 |     100 |     100 |                                         
  utf8.js        |     100 |      100 |     100 |     100 |                                         
-----------------|---------|----------|---------|---------|-----------------------------------------

CC/ @alanshaw @Gozala