Move @actions/http-client into the toolkit (#1062)

💡 See https://github.com/actions/toolkit/pull/1064 for a better diff!

https://github.com/actions/toolkit contains a variety of packages used for building actions.  https://github.com/actions/http-client is one such package, but lives outside of the toolkit.  Moving it inside of the toolkit will improve discoverability and reduce the number of repos we have to keep track of for maintenance tasks (such as github/c2c-actions-service#2937).

I checked with @bryanmacfarlane on the historical decision here.  Apparently it was just inertia from before we released the toolkit as multiple packages.

The benefits here are:
- Have one fewer repo to keep track of
- Signal that this is an HTTP client meant for building actions, not for general use.

## Notes
- `@actions/http-client` will continue to be released as its own package.
- Bumping the package version to **2.0.0**.  Since we're compiling in strict mode now, there are some breaking changes to the exported types.  This is an improvement because the null-unsafe version of`http-client` is currently breaking the safety of null-safe consumers.
- I'm not updating the other packages to use the new version in this PR.  I plan to do that in a follow-up.  We'll hold off on publishing `http-client` v2 to NPM until that's done just in case other changes shake out of it.

packages/http-client/__tests__/keepalive.test.ts

@@ -0,0 +1,73 @@
+import * as httpm from '../lib'
+
+describe('basics', () => {
+  let _http: httpm.HttpClient
+
+  beforeEach(() => {
+    _http = new httpm.HttpClient('http-client-tests', [], {keepAlive: true})
+  })
+
+  afterEach(() => {
+    _http.dispose()
+  })
+
+  it('does basic http get request with keepAlive true', async () => {
+    const res: httpm.HttpClientResponse = await _http.get(
+      'http://httpbin.org/get'
+    )
+    expect(res.message.statusCode).toBe(200)
+    const body: string = await res.readBody()
+    const obj = JSON.parse(body)
+    expect(obj.url).toBe('http://httpbin.org/get')
+  })
+
+  it('does basic head request with keepAlive true', async () => {
+    const res: httpm.HttpClientResponse = await _http.head(
+      'http://httpbin.org/get'
+    )
+    expect(res.message.statusCode).toBe(200)
+  })
+
+  it('does basic http delete request with keepAlive true', async () => {
+    const res: httpm.HttpClientResponse = await _http.del(
+      'http://httpbin.org/delete'
+    )
+    expect(res.message.statusCode).toBe(200)
+    const body: string = await res.readBody()
+    JSON.parse(body)
+  })
+
+  it('does basic http post request with keepAlive true', async () => {
+    const b = 'Hello World!'
+    const res: httpm.HttpClientResponse = await _http.post(
+      'http://httpbin.org/post',
+      b
+    )
+    expect(res.message.statusCode).toBe(200)
+    const body: string = await res.readBody()
+    const obj = JSON.parse(body)
+    expect(obj.data).toBe(b)
+    expect(obj.url).toBe('http://httpbin.org/post')
+  })
+
+  it('does basic http patch request with keepAlive true', async () => {
+    const b = 'Hello World!'
+    const res: httpm.HttpClientResponse = await _http.patch(
+      'http://httpbin.org/patch',
+      b
+    )
+    expect(res.message.statusCode).toBe(200)
+    const body: string = await res.readBody()
+    const obj = JSON.parse(body)
+    expect(obj.data).toBe(b)
+    expect(obj.url).toBe('http://httpbin.org/patch')
+  })
+
+  it('does basic http options request with keepAlive true', async () => {
+    const res: httpm.HttpClientResponse = await _http.options(
+      'http://httpbin.org'
+    )
+    expect(res.message.statusCode).toBe(200)
+    await res.readBody()
+  })
+})