helmet: Add featurePolicy

[tomi]

feture-policy provides TS typings with it, but it doesn't export the
interface for the options parameter. We could have used conditional
typings and infered the type of the parameter, but that would have
restricted the minimum TS version to 2.8, so instead I created the
interface definition here.

Resolves #37627

Please fill in this template.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

Select one of these and delete the others:

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: https://helmetjs.github.io/docs/feature-policy/
• If this PR brings the type definitions up to date with a new version of the JS library, update the version number in the header.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }. If for reason the any rule need to be disabled, disable it for that line using // tslint:disable-next-line [ruleName] and not for whole package so that the need for disabling can be reviewed.

[typescript-bot]

@tomi Thank you for submitting this PR!

🔔 @cyrilschumacher @EvanHahn @BlueHatbRit @chdanielmueller - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

👋 Hi there! I’ve run some quick measurements against master and your PR. These metrics should help the humans reviewing this PR gauge whether it might negatively affect compile times or editor responsiveness for users who install these typings.

Let’s review the numbers, shall we?

Comparison details 📊

	master	#39673	diff
Batch compilation
Memory usage (MiB)	69.3	65.6	-5.4%
Type count	9218	9228	0%
Assignability cache size	1057	1062	0%
Language service
Samples taken	323	342	+6%
Identifiers in tests	323	342	+6%
getCompletionsAtPosition
Mean duration (ms)	380.1	382.9	+0.7%
Mean CV	12.0%	11.9%
Worst duration (ms)	462.6	497.8	+7.6%
Worst identifier	setTo	helmet
getQuickInfoAtPosition
Mean duration (ms)	396.1	397.2	+0.3%
Mean CV	12.9%	12.6%	-2.1%
Worst duration (ms)	487.8	521.5	+6.9%
Worst identifier	hpkpTest	maxAge

It looks like nothing changed too much. I won’t post performance data again unless it gets worse.

[EvanHahn]

I don't exactly understand why we can't use the built-in types from the feature-policy module, but I trust that it's the case. Looks good to me.

[typescript-bot]

A definition owner has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!

[typescript-bot]

I just published @types/helmet@0.0.45 to npm.