Class OAuth1aSigner

Builds the full Authorization: OAuth ... header value for a single request, computing the HMAC signature over the canonical signature base string.

Spec steps implemented (RFC 5849 §3.4):

1. Collect oauth_* params + the request's query params.
2. Percent-encode, sort, and &-join them (the normalized parameter string).
3. Build the base string METHOD&encode(baseURL)&encode(params).
4. Sign with key encode(consumerSecret)&encode(tokenSecret).
5. Emit OAuth realm="...", oauth_consumer_key="...", ..., oauth_signature="...".