feat: some basic samples, plus two new exports

[alexander-fenster]

So... I did not expect that gax will ever get any kind of samples, but I just found that I've been using these two code snippets to explain how createApiCall works, and samples/ folder is probably a good place for them. For any new team member, this might be a good place to go to understand what that weird piece of code in the client libraries actually does:

gax-nodejs/test/fixtures/google-gax-packaging-test-app/src/v1beta1/echo_client.js

Lines 207 to 213 in e66e0f1

	this._innerApiCalls[methodName] = gaxModule.createApiCall(
	innerCallPromise,
	defaults[methodName],
	this._descriptors.page[methodName] ||
	this._descriptors.stream[methodName] ||
	this._descriptors.longrunning[methodName]
	);

If we get lucky, I might find some time to showcase streaming, long running operations, and createStub in the same way.

Re: license change - the whole package uses BSD 3-clause, the samples package was empty, so I see no problem with changing the license to BSD to match the other files in the package.

Re: new exports - just some useful functions which, for some reasons, were never exported.

[codecov]

Codecov Report

Merging #702 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #702      +/-   ##
=========================================
+ Coverage   89.68%   89.9%   +0.22%     
=========================================
  Files          41      43       +2     
  Lines        7522    7690     +168     
  Branches      407     407              
=========================================
+ Hits         6746    6914     +168     
  Misses        773     773              
  Partials        3       3

Impacted Files	Coverage Δ
src/index.ts	97.12% <100%> (+0.08%)	⬆️
samples/pagination.js	100% <100%> (ø)
samples/quickstart.js	100% <100%> (ø)

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