Add environment variable switch for certificates.

[LeonLiuY]

Since the DefaultClient is created at package initialize phase, the only way to put this switch is environment variable.

[mattrobenolt]

Could we not expose a method to set them? I imagine people also wanted to just explicitly give the certs to use. So something like, client.SetRootCA() or something.

[LeonLiuY]

client.Transport is an interface with only 1 method Send. I have 2 choices:

• cast it to http.Transport.
• recreate another http.Transport.

Neither is good smell to me. But I implemented the latter.

PR is updated

[kwargs]

Hi @mattrobenolt

Do you have any plans to merge this PR?
This PR is very helpful with self-signed-ceritificates.

[dcramer]

If we're still committed to this being a good (enough) implementation I'm happy to get this merged. In general it seems fine to me, and is probably nicer than an env variable. I do know the env variable pattern does exist elsewhere, but usually its for general "load certs from here".