Allow setting certificates via application config

[josevalim]

This pull request allows setting a custom location for certificates via the application config.

Currently we are seeing a proliferation of env vars for reading certificates from in the Elixir community. Here are some examples:

• https://github.com/hexpm/hex/blob/eec7a266f6e1b1c754798ee9a9c17b4b6201fff2/lib/hex/state.ex#L111
• https://github.com/elixir-lang/elixir_make/blob/67ef8c1e249d1562fee9247320a602908f0094c6/lib/elixir_make/downloader/httpc.ex#L53
• https://github.com/elixir-nx/bumblebee/blob/b01e0da989a39b594990f8023bebb3751663fb19/lib/bumblebee/utils/http.ex#L191

This means anyone using Elixir/Erlang behind a proxy needs to setup several env vars, carefully reading the docs of each package that may do external HTTP requests.

For this reason, I believe a solution upstream in Erlang itself would be better, as it would avoid the same issue (of several multiple env vars or application config) happening in both Erlang and Elixir packages, escripts, etc.

I understand it is possible to call public_key:cacerts_load/1 but I don't believe it fully solves the problem:

1. If we leave it up for each package to call public_key:cacerts_load/1, then we land in the same problem described here, but even worse, as one package would globally override the defaults of others

2. If we leave it up for users of the packages to call it, then it means they need to add logic to each of their applications and packages that they use. And doing so for escripts is even harder.

This is a follow up to #8874.

[github-actions[bot]]

CT Test Results

  2 files   17 suites   5m 55s :stopwatch: 284 tests 282 :white_check_mark: 2 :zzz: 0 :x: 300 runs  298 :white_check_mark: 2 :zzz: 0 :x:

Results for commit 13d6a352.

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// Erlang/OTP Github Action Bot

[dgud]

Can you add one testcase for regression testing at least.

[josevalim]

I have augmented the existing tests!

[dgud]

Can you rebase this to maint so I can include it in 27.2?

[josevalim]

@dgud done!