feat: Add ability to set proxy

[eitoball]

Fixes #70

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md example file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• This PR should add ability to set proxy when initializing Sendgrid::Client.

Sample

host = 'https://example.com'
headers = { 'Authorization' => 'Bearer xxx' }
proxy_options = { host: 'localhost', port: 8080, user: 'anonymous', pass: 'secret' }
client = SendGrid::Client.new(host: host, request_headers: headers, proxy_options: proxy_options }
res = client.version('v3').get

[SendGridDX]

All committers have signed the CLA.

[kplimack]

what about https_proxy ?

[eitoball]

No, you cannot use https_proxy. It is from ruby's `URI::Generic#find_proxy'.

[kplimack]

@eitoball I'm not sure that it is a safe idea for the client code to mutate environment variables. If one of those is set the client should respect it. It makes more sense to instantiate the class like you have in example#1 passing the proxy settings explicitly.

[eitoball]

@kplimack I am not sure what you mean by "mutate environment variables". Example (2) in examples/example.rb is just showing that you can use http_proxy environment variable to set proxy. The environment variable can be set in shell like export http_proxy=127.0.0.1:8080.

[thinkingserious]

Hello @eitoball,

Thanks again for the PR!

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[thinkingserious]

Hello @eitoball,

Thanks again for the PR!

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[thinkingserious]

Hello @eitoball,

Thanks again for the PR!

You have earned 3 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

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Thank you and Happy Hacktobering!

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