feat: allow to specify encoding strategy for query params

[RudolfMan]

I was adapting Tesla in a project when we encode query params with :rfc3986 and I noticed that, while Tesla provides conveniences to build url encoding query params for us, it doesn't allow to specify the encoding strategy.

TL;DR

This PR allows specifying option query_encoding: :rfc3986 so that whitespaces in query params will be encoded as "%20".

Tesla.get("http://example.com", query: [username: "John Smith"], opts: [query_encoding: :rfc3986])

will build the following url

http://example.com?username=John%20Smith

Also adds optional argument to Tesla.build_url and Tesla.encode_query.

---

Tesla uses URI.encode_query/2 to encode query params. Since Elixir 1.12 that function allows to specify the encoding strategy.

You can specify one of the following encoding strategies:

• :www_form - (default, since v1.12.0) keys and values are URL encoded as per encode_www_form/1. This is the format typically used by browsers on query strings and form data. It encodes " " as "+".

• :rfc3986 - (since v1.12.0) the same as :www_form except it encodes " " as "%20" according RFC 3986. This is the best option if you are encoding in a non-browser situation, since encoding spaces as "+" can be ambiguous to URI parsers. This can inadvertently lead to spaces being interpreted as literal plus signs.

Encoding defaults to :www_form for backward compatibility.

[yordis]

Related to: #538

[yordis]

Thank you so much for taking the time for this. I appreciate it.

[yordis]

Any final thoughts related to our conversation?

[yordis]

@teamon share some love over here! Is the function changing from arity 2 to arity 3 with the default value any concern here?

[teamon]

This is all backwards compatible, looks good to me!