edit: add --field to edit a single field

[NanShanFish]

Feature

Add --field to rbw edit, matching rbw get --field. See #365.

Currently rbw edit can only edit password + notes; this lets users edit a
single field (username, name, notes, password, totp, uris, or a custom field)
from the CLI.

Depends on #366 (editing keyed entries must encrypt with the item key and send
the key field, so this branch is based on that fix).

Changes

• src/bin/rbw/main.rs: add -f/--field to the Edit subcommand
• src/bin/rbw/commands.rs:
  • split edit into edit_default (existing password + notes flow) and
    edit_field (single-field flow)
  • edit_field: open the editor with the current plaintext value -> parse via
    parse_field_value -> encrypt with the entry key (org / item key) -> update
    the field -> PUT
  • supported fields: username, name, notes, password (records old password
    into history), totp, uris (one per line, preserving existing per-URI match
    types by plaintext match), custom fields (matched by name substring)
  • empty input clears the field; a clear error is raised for fields that are
    not available on the entry type
• new parse_field_value helper with unit tests

Usage

rbw edit --field username <entry>                       # editor-based
echo "newuser" | rbw edit --field username <entry>      # non-interactive

Testing

• cargo test (all targets) passes
• cargo fmt / cargo clippy --all-targets clean

Fixes #365

[devgar]

Hi! I found that editing custom fields always fails on this branch (names are compared while still encrypted), and opened a PR against your branch with a fix plus a few related improvements: NanShanFish#1. Tested against a real vault. Hope it helps get this merged!