feat: override adapter at OTP app level

[c4710n]

Thanks for all your hard works, Tesla already has great support on choosing adapter at different levels.

But, it still have one case that tesla doesn't support - override adapter at OTP app level.

This PR adds that feature, which is useful when we want to use different adapter for different OTP apps.

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An example - I installed two packages from Hex:

• foo, whose OTP app name is :foo, the underlying HTTP client is Tesla
• bar, whose OTP app name is :bar, the underlying HTTP client is Tesla, too.

I want to use :finch as the adapter for them all, but due to some reason, I have to set HTTP proxy for bar. But, :finch doesn't support proxy, for now. So I have to use :hackney, which supports proxy.

With this PR, I can do something like:

config :telsa, :adapter, {Tesla.Adapter.Finch, name: MyFinch}
config :tesla, :bar, adapter: Tesla.Adapter.Hackney # override adapter for :bar in an easy way.

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Feel free to comment. I can make any modifications necessary to ensure this PR is mergeable. ;)