feat: [SIW-860,SIW-946] Add Android integrity implementation

[LazyAfternoons]

Important

Depends on #6

Short description

This PR adds the Android implementation to retrive a key attestation and to generate a Play Integrity standard request.
It also implements a basic backend server to verify these attestations.

List of Changes

Adds Android native implementation of the following functions:

• isPlayServicesAvailable which checks whether or not Google Play Services are available on the device. They are request for Play Integrity API to work, along with Play Store;
• prepareIntegrityToken is the first function which must be called for a Play Integrity standard request. It prepares the provider which is later used to obtain the integrity token. A Google Cloud Project Number is required and must be provided in the const declaration of AndroidApp.tsx;
• requestIntegrityToken is the second function to be called for a Play Integrity standard request. A requestHash can be provided and will be later checked from the server which decrypts the integrity token;
• getAttestation returns the key attestation which will be later checked from the server.

Adds backend endpoints implementation of:

• /verifyIntegrityToken which verifies and decodes a Google Play Integrity integrity token and returns its verdict;
• /verifyAttestation which verifies the attestation by checking if the root certificate is signed by Google; if each certificate in the chain is signed by the previous one; if there's any revoked certificate in the chain. Each step is implemented separately for clarity's sake, at the cost of cycling through the chain multiple times.

The AndroidApp.tsx component of the example app incapsulates the whole UI needed to test these function and thus is only rendered if the platform is Android.
On iOS these functions are rejected by default.

The androidRouter.ts component of the backend exposes any Android related endpoint.

How Has This Been Tested?

Run the Android example app by following the instructions in the README.md.

You can test the generated tokens and attestation with the local backend. Read the README.md for more information.

The flow to test a Google Play Integrity check is:

• Prepare;
• Get Token;
• Verify Token (optional, requires the dev server which only decodes the results and sends it to the example app).

The flow for the key attestation is:

• Get attestation;
• Verify attestation (optional, requires the dev server which performs some checks on the chain).

Contact me in private and I'll share my Google Cloud credentials to facilitate the review process.

[shadowsheep1]

LGTM

[dpulls]

🎉 All dependencies have been resolved !