ARROW-8064: [Dev] Implement Comment bot via Github actions

See it in action: kszucs/arrow#16 (comment)

Main drawback that is is much slower than ursabot, but we can optimize it by:
- porting crossbow to only depend on pygithub instead of libgit2 (will consume the rate limit, but should fit in)
- use caching or docker

Theoretically CROSSBOW_GITHUB_TOKEN is set as a github actions secret, see https://issues.apache.org/jira/browse/INFRA-19954
We can trigger a build once this is merged into master.

Closes #6571 from kszucs/master and squashes the following commits:

7a604a875 <Krisztián Szűcs> note that the license is BSD2
8586eb727 <Krisztián Szűcs> add license reference
def872434 <Krisztián Szűcs> RAT
a96e7e24c <Krisztián Szűcs> flake8
6f5da639d <Krisztián Szűcs> add requirements to docker whitelist
6678c2e0a <Krisztián Szűcs> update archery dependencies
33f65d48c <Krisztián Szűcs> revert removing the rest of the workflows
a82b8790a <Krisztián Szűcs> test dep
06a7716cb <Krisztián Szűcs> responses test dep
ba2522989 <Krisztián Szűcs> fix archery workflow syntax
9352ee05c <Krisztián Szűcs> run archery unittests
deb857ff1 <Krisztián Szűcs> checkout@v2 and fetch tags
215495a3d <Krisztián Szűcs> fix result path
748832f75 <Krisztián Szűcs> message formatter
ea1b7c863 <Krisztián Szűcs> no dry run
6c83b0c40 <Krisztián Szűcs> dry run
4789ac5d5 <Krisztián Szűcs> response ormatter
1b0b15d5a <Krisztián Szűcs> cleanup
2270a35a9 <Krisztián Szűcs> validate
035024fa0 <Krisztián Szűcs> validate callback
e791c627b <Krisztián Szűcs> diag
641227f73 <Krisztián Szűcs> diab
b22b20400 <Krisztián Szűcs> token
d95e86b7b <Krisztián Szűcs> path to event payload
3e9a27909 <Krisztián Szűcs> pygithub
ca1592d5d <Krisztián Szűcs> typo
3c1358eff <Krisztián Szűcs> triger event handler
55e65faf3 <Krisztián Szűcs> crossbow command
92568eb5e <Krisztián Szűcs> first draft of bot
99ea0c2b5 <Krisztián Szűcs> cat
3c0f16d83 <Krisztián Szűcs> remove all other workflows
1f8f21de9 <Krisztián Szűcs> diag event handling
2f613dd15 <Krisztián Szűcs> Check event handling (#15)

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>

LICENSE.txt

@@ -967,6 +967,36 @@ OR OTHER DEALINGS IN THE SOFTWARE.
 
 --------------------------------------------------------------------------------
 
+The projects includes code from the Ursabot project under the dev/archery
+directory.
+
+License: BSD 2-Clause
+
+Copyright 2019 RStudio, Inc.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+--------------------------------------------------------------------------------
+
 This project include code from CMake.
 
 * cpp/cmake_modules/FindGTest.cmake is based on code from CMake.