add cookie support

usage:
 - create cookies.json file somewhere, preferrably outside cobalt directory
 - in docker, you can bind mount it (`volumes` in composefile)
   - if you don't want cobalt to update the cookies, set it to `:ro` (cobalt will print a warning about this, ignore it)
 - set COOKIE_PATH to the absolute path of this file
 - enjoy?

usage in services: probably the simplest api ever
 - import { getCookie, updateCookie } from '../../cookie/manager.js';
 - const cookie = getCookie('<service_name>');
   - add this to headers - `headers: { cookie }`
 - after fetch is done, save potential cookie updates: updateCookie(cookie, fetch.headers)
 - see instagram.js for example usage

src/modules/cookie/manager.js

@@ -0,0 +1,78 @@
+import path from 'path'
+import Cookie from './cookie.js'
+import { readFile, writeFile } from 'fs/promises'
+import { parse as parseSetCookie, splitCookiesString } from 'set-cookie-parser'
+
+const WRITE_INTERVAL = 60000,
+      COOKIE_PATH = process.env.COOKIE_PATH,
+      COUNTER = Symbol('counter');
+
+let cookies = {}, dirty = false, intervalId;
+
+const setup = async () => {
+    try {
+        if (!COOKIE_PATH)
+            return
+
+        cookies = await readFile(COOKIE_PATH, 'utf8')
+        cookies = JSON.parse(cookies)
+        intervalId = setInterval(writeChanges, WRITE_INTERVAL)
+    } catch { /* no cookies for you */ }
+}
+
+setup()
+
+function writeChanges() {
+    if (!dirty) return
+    dirty = false
+
+    writeFile(
+        COOKIE_PATH,
+        JSON.stringify(cookies, null, 4)
+    ).catch(e => {
+        console.error('warn: cookies failed to save, stopping interval')
+        console.error('exception:', e)
+        clearInterval(intervalId)
+    })
+}
+
+export function getCookie(service) {
+    if (!cookies[service] || !cookies[service].length)
+        return
+
+    let n
+    if (cookies[service][COUNTER] === undefined) {
+        n = cookies[service][COUNTER] = 0
+    } else {
+        ++cookies[service][COUNTER]
+        n = (cookies[service][COUNTER] %= cookies[service].length)
+    }
+
+    const cookie = cookies[service][n]
+    if (typeof cookie === 'string')
+        cookies[service][n] = Cookie.fromString(cookie)
+
+    return cookies[service][n]
+}
+
+// todo: expiry checking? domain checking?
+// might be pointless for the purposes of cobalt
+export function updateCookie(cookie, headers) {
+    const parsed = parseSetCookie(
+        splitCookiesString(headers.get('set-cookie'))
+    ), values = {}
+
+    cookie.unset(
+        parsed
+            .filter(c => c.expires < new Date())
+            .map(c => c.name)
+    )
+
+    parsed
+        .filter(c => c.expires > new Date())
+        .forEach(c => values[c.name] = c.value);
+
+    cookie.set(values)
+    if (Object.keys(values).length)
+        dirty = true
+}