doc: COSE -> JWK Mappings

**Note**:
- **Browser** WebCrypto only supports **P-256**
- https://caniuse.com/mdn-api_subtlecrypto_sign_ed25519
- **Node** `node:crypto` and WebCrypto also supports **Ed25519** and **secp256k1** (as just `SHA256`) - whatever is supported by openssl v3
- https://nodejs.org/docs/latest/api/webcrypto.html#algorithm-matrix
- https://nodejs.org/docs/latest/api/crypto.html#cryptocreatesignalgorithm-options
- `openssl list -digest-algorithms` (TL;DR: always use `SHA256`)
- `openssl ecparam -list_curves` (`secp256k1`, `prime256v1: X9.62/SECG`, a.k.a `secp256r1` a.k.a. `P-256`)
- `openssl list -public-key-algorithms` (`1.3.101.112, ED25519`, `1.2.840.10045.2.1, EC, id-ecPublicKey`)

#### Node WebCrypto Key Gen Example

```js
'use strict';

let Fs = require('node:fs/promises');

async function generateKeyPair(algo) {
let keyPair = await crypto.subtle.generateKey(algo, true, ['sign', 'verify']);

let privateKeyAb = await crypto.subtle.exportKey('pkcs8', keyPair.privateKey);
let privateKeyDER = new Uint8Array(privateKeyAb);
let privateKeyPath = `./key.${algo.name}.der`.toLowerCase();
await Fs.writeFile(privateKeyPath, privateKeyDER);

let publicKeyAb = await crypto.subtle.exportKey('spki', keyPair.publicKey);
let publicKeyDER = new Uint8Array(publicKeyAb);
let publicKeyPath = `./pub.${algo.name}.der`.toLowerCase();
await Fs.writeFile(publicKeyPath, publicKeyDER);

console.info(`Key pair generated and saved as ${privateKeyPath} and ${publicKeyPath}`);
}

async function main() {
await generateKeyPair({ name: 'Ed25519' });
await generateKeyPair({ name: 'ECDSA', namedCurve: 'P-256' });
}

main().catch(function (err) {
console.error(err.stack);
process.exit(1);
});
```

Adapted From

### OKP (EdDSA / ed25519)

| Key Type | Name | COSE Label | Type | Description | Reference |
| --- | --- | --- | --- | --- | --- |
| 1 | crv | -1 | int / tstr | EC identifier | [RFC9053] |
| 1 | x | -2 | bstr | Public Key | [RFC9053] |
| 1 | d | -4 | bstr | Private key | [RFC9053] |

### EC (ECDSA, secp256k1)

| Key Type | Name | COSE Label | Type | Description | Reference |
| --- | --- | --- | --- | --- | --- |
| 2 | crv | -1 | int / tstr | EC identifier | [RFC9053] |
| 2 | x | -2 | bstr | x-coordinate | [RFC9053] |
| 2 | y | -3 | bstr / bool | y-coordinate | [RFC9053] |

### Modern Keypairs

These are the key types that are still relevant in 2024+ and will be relevant for the next decade:

```js
let coseMap = {
"keys": {
"kty": 1,
"alg": 3
},
"okp": {
"crv": "-1",
"x": "-2",
"d": "-4"
},
"ec": {
"crv": "-1",
"x": "-2",
"y": "-3",
"d": "-4"
},
"kty": {
"1": "OKP",
"2": "EC"
},
"alg": {
"-47": "ES256K",
"-8": "EdDSA",
"-7": "ES256"
},
"crv": {
"1": "P-256",
"6": "Ed25519",
"8": "secp256k1"
}
};
```

### Legacy Keypairs

```js
let coseMap = {
"keys": {
"kty": 1,
"alg": 3
},
"okp": {
"crv": "-1",
"x": "-2",
"d": "-4"
},
"ec": {
"crv": "-1",
"x": "-2",
"y": "-3",
"d": "-4"
},
"rsa": {
"n": "-1",
"e": "-2",
"d": "-3",
"p": "-4",
"q": "-5",
"dp": "-6",
"dq": "-7",
"qi": "-8"
},
"kty": {
"1": "OKP",
"2": "EC",
"3": "RSA"
},
"alg": {
"-259": "RS512",
"-258": "RS384",
"-257": "RS256",
"-39": "PS512",
"-38": "PS384",
"-37": "PS256",
"-36": "ES512",
"-35": "ES384",
"-47": "ES256K",
"-8": "EdDSA",
"-7": "ES256"
},
"crv": {
"1": "P-256",
"2": "P-384",
"3": "P-521",
"6": "Ed25519",
"8": "secp256k1"
}
};
```

### RSA (legacy)

RSA's not bad or broken, it's just not efficient.

| Key Type | Name | COSE Label | Type | Description | Reference |
| --- | --- | --- | --- | --- | --- |
| 3 | n | -1 | bstr | RSA modulus `n` | [RFC8230] |
| 3 | e | -2 | bstr | RSA public exponent `e` | [RFC8230] |
| 3 | d | -3 | bstr | RSA private exponent `d` | [RFC8230] |
| 3 | p | -4 | bstr | Prime factor `p` of `n` | [RFC8230] |
| 3 | q | -5 | bstr | Prime factor `q` of `n` | [RFC8230] |
| 3 | dP | -6 | bstr | `dP` is `d` mod (`p - 1`) | [RFC8230] |
| 3 | dQ | -7 | bstr | `dQ` is `d` mod (`q - 1`) | [RFC8230] |
| 3 | qInv | -8 | bstr | CRT coefficient `q^(-1)` mod `p` | [RFC8230] |
| 3 | other | -9 | array | Other prime infos, an array | [RFC8230] |
| 3 | r_i | -10 | bstr | Prime factor `r_i` of `n`, where `i >= 3` | [RFC8230] |
| 3 | d_i | -11 | bstr | `d_i = d` mod (`r_i - 1`) | [RFC8230] |
| 3 | t_i | -12 | bstr | CRT coefficient `t_i = (r_1 * r_2 * ... * r_(i-1))^(-1)` mod `r_i` | [RFC8230] |