feat: attr_get / attrs_get attribute accessors (1.4.0)

[andrewstarks]

Summary

Adds two module-level accessors that look up decomposed attributes by name on a block (a `doc.session` or any `doc.media[i]`):

• `sdp.attr_get(block, name)` — first matching attribute table, or `nil`
• `sdp.attrs_get(block, name)` — array of all matches in document order (empty table when none)

Both are nil-safe and mirror `parse_sdp.grammar.base.params_get` (the inner-`fmtp` accessor) for the outer `attributes` array, so consumers no longer hand-roll the scan over the ordered array. `attrs_get` earns its place because same-name attributes are common (`rtpmap` / `fmtp` / `ssrc` / `rtcp-fb`).

Requested by the `lnmos` projection layer, which was the only consumer still hand-rolling `find_attr`.

Design

• Defined once in `grammar/base.lua` beside `params_get`, inherited through `extend()`, re-exported on the public module in `init.lua`.
• Free functions, not block methods — works identically on parsed and `sdp.new()` docs with no metatable surface on blocks, and no impact on `to_json()` / round-trip.

Tests

+9 cases in `spec/library_spec.lua` (first / all / session-level / flag-only / miss / nil-safe / hand-built). Full hermetic suite green.

Release

Bumps rockspec to 1.4.0 (additive, backward-compatible). dkjson dependency unchanged.

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