Add Terminal Server Client username parser

[wwwesh]

Parse cached username hints stored by the Microsoft Terminal Server
Client for previously entered server names.

These registry values represent cached GUI input and do not prove that
an RDP connection or connection attempt occurred. Analysts should
corroborate the results with other artifacts, such as Windows event
logs. No assumption is made about the retention limit of the Servers
key.

Proposed Changes

• Add a parser for
  HKCU\Software\Microsoft\Terminal Server Client\Servers.
• Parse server names and their cached UsernameHint values.
• Return the registry timestamp, user metadata, and registry metadata.
• Skip server entries without a UsernameHint value.
• Document the forensic interpretation limitations of this artifact.
• Add tests for valid entries, missing values, and incompatible targets.

Testing

$ uv run --no-sync pytest tests/plugins/os/windows/regf/test_terminalserverclient.py -q
3 passed 

$ uv run --no-sync pytest tests/plugins/os/windows/regf -q
27 passed 

$ uv run --no-sync ruff check dissect/target/plugins/os/windows/regf/terminalserverclient.py tests/plugins/os/windows/regf/test_terminalserverclient.py
All checks passed!

$ uv run --no-sync ruff format --check dissect/target/plugins/os/windows/regf/terminalserverclient.py tests/plugins/os/windows/regf/test_terminalserverclient.py
2 files already formatted

Checklist

• PR Title is descriptive of the changes
• The description is descriptive of the changes
• Tests are included and pass
• Closes Registry Terminal Server Client username parser #1450

Closes #1450