Enhance account creation and configuration options

[hesham-fouda]

Summary

Two additive enhancements that make the SDK reusable against any mail.tm-compatible API (e.g. self-hosted instances)
and give callers more control over how disposable accounts are generated.

Both changes are fully backwards-compatible — every existing call site keeps working unchanged.

Changes

1. Configurable baseUrl, baseMercure, and custom headers

The constructor now accepts three optional fields:

Option	Purpose
baseUrl	API base. Defaults to https://api.mail.tm.
baseMercure	Mercure SSE hub. Defaults to https://mercure.mail.tm/.well-known/mercure.
headers	Record<string, string> merged into every request. Useful for x-api-key, custom auth, tracing IDs,
etc.

const mailjs = new Mailjs({
  baseUrl:     "https://api.example.com",
  baseMercure: "https://mercure.example.com/.well-known/mercure",
  headers: { "x-api-key": "xxxx" },
});

baseUrl / baseMercure were previously hard-coded private fields, so this is the first way to point the SDK at a
non-mail.tm host without forking.

2. Richer options on createOneAccount

createOneAccount() now accepts an options object alongside the legacy useUUID: boolean form:

┌──────────┬───────────────────────────────────────────────────────────────────────┐
│  Option  │                                Effect                                 │
├──────────┼───────────────────────────────────────────────────────────────────────┤
│ domain   │ Use this domain directly — skips the getDomains() round-trip.         │
├──────────┼───────────────────────────────────────────────────────────────────────┤
│ name     │ Exact local-part. Overrides prefix / hashSize / useUUID.              │
├──────────┼───────────────────────────────────────────────────────────────────────┤
│ prefix   │ Local-part prefix; a random hex suffix of hashSize bytes is appended. │
├──────────┼───────────────────────────────────────────────────────────────────────┤
│ hashSize │ Random suffix size in bytes (= 2× hex chars). Default 8.              │
├──────────┼───────────────────────────────────────────────────────────────────────┤
│ password │ Override the auto-generated password.                                 │
├──────────┼───────────────────────────────────────────────────────────────────────┤
│ useUUID  │ Same as before; ignored when name is set.                             │
└──────────┴───────────────────────────────────────────────────────────────────────┘

Local-part precedence: name > prefix+hashSize > useUUID > random hex.

// Legacy form — still works
await mailjs.createOneAccount();
await mailjs.createOneAccount(true);

// New options
await mailjs.createOneAccount({ domain: "yourdomain.com" });
await mailjs.createOneAccount({ domain: "yourdomain.com", prefix: "test-", hashSize: 4 });
await mailjs.createOneAccount({ domain: "yourdomain.com", name: "support", password: "s3cret-pw" });

The domain option is the practical motivator — server-to-server flows that already know the target domain no longer
need to hit /domains on every account creation.

Compatibility

- No breaking changes. All existing call sites behave exactly as before when none of the new options are passed.
- IRequestObject.headers typing is unchanged at the public surface; extra headers are merged via Object.assign inside
_send so no public type widens.
- Tests in tests.js continue to pass against the live mail.tm service.

Test plan

- Existing tests pass (npm run build && node tests.js)
- new Mailjs() with no args still hits api.mail.tm
- headers: { "x-api-key": "..." } is sent on every request
- createOneAccount({ domain }) skips getDomains() (verified by network logs)
- createOneAccount({ name }) produces the exact local-part
- createOneAccount(true) still produces a UUID local-part