Support name-addr Refer-To headers

[yeoleobun]

What changed

This updates the dialog refer() APIs to accept full Refer-To header values instead of only Uri inputs.

• widen refer() to accept impl Into<crate::sip::ReferTo> across dialog types
• add From<crate::sip::Uri> for ReferTo so existing URI callers still serialize as <sip:...>
• add focused tests for both URI compatibility and raw name-addr input

Why it changed

Issue #115 points out that RFC 3515 allows Refer-To to carry the full name-addr form, including an optional display name and parameters. The previous API only accepted Uri and always forced Refer-To into "<uri>", which prevented callers from supplying valid name-addr values.

Impact

• existing callers that pass Uri continue to work unchanged
• callers can now pass a full Refer-To value such as "Display Name" <sip:user@domain.com;method=INVITE?Replaces=...>
• downstream REFER-related tests in sibling repos continue to pass against this local branch

Root cause

The REFER helpers modeled Refer-To as a URI-only input even though the header syntax is wider than a bare URI.

Validation

• cargo test --lib
• cargo test refer in active-call
• cargo test test_inbound_refer_success --lib in rustpbx
• cargo test --test rwi_transfer_refer_e2e_test in rustpbx

Closes #115.