feat: Extend client.setDefault[Request,Headers] to accept objects

[aslafy-z]

Fixes #642

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 file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Accept objects for modifying defaultRequest and defaultHeaders

eg: Configure request to use a sock proxy (#642 use case):

const client = require('@sendgrid/client');
const Agent = require('socks5-http-client/lib/Agent');

client.setDefaultRequest({
	agentClass: Agent,
	agentOptions: {
		socksHost: 'localhost',
		socksPort: 5555,
	},
});

// instead of

client.setDefaultRequest('agentClass', Agent);
client.setDefaultRequest('agentOptions', {
	socksHost: 'localhost',
	socksPort: 5555,
});

[aslafy-z]

Not sure with the last commit, I'm new to typescript, would love a review. Thank you!

[thinkingserious]

Hello @aslafy-z,

Thanks again for the PR!

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[Ditofry]

🕺 🎊 Thanks!

[SendGridDX]

All committers have signed the CLA.

[childish-sambino]

LGTM