added data_tailscale_acl data source

[markwellis]

What this PR does / why we need it:
there is currently no acl data_source which makes working with the acl hard

Which issue this PR fixes (use fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):

Fixes #180

Special notes for your reviewer:
I decided to expose it as a json blob as writing a schema and mangling it into that schema at load time was a lot of work and I don't see the benefit.
resource "tailscale_acl" takes a json string so it doesn't seem unreasonable that the data source for that would give a json string too
Also the external script I've been using up till now also works by providing a json string, which makes this a drop in replacement for that.

It works quite well with this test

data "tailscale_acl" "acl" {}

locals {
  acl = jsondecode(data.tailscale_acl.acl.json)
}

resource "tailscale_acl" "acl" {
  acl = jsonencode(merge(local.acl, {groups = merge(local.acl.groups, {"group:foo" = ["br@asd.com"]})}))

  allow_overwrite = true # in #303 
}

P.s. sorry if the code's not great, it's the first go I've written so I have no idea about best practice etc

[knyar]

Thank you! Just a couple of minor comments here.

[markwellis]

anything you need from me to get this moving?

thanks

[knyar]

anything you need from me to get this moving?

It seems that CI checks are still failing. In this case I think what's missing is running go generate (or tfplugindocs directly) to regenerate markdown docs. Thank you!

[markwellis]

I have updated the PR with tfplugindocs

[knyar]

Thank you!